Here is a good place to start as far as sump/refugium size:
http://www.melevsreef.com/make_a_sump.html
Generally the larger the better, but you would be fine with something around 30 gallons.
Your return pump doesn't really have to be that powerful. It depends on how much water your overflow drains because you can only pump back in as fast as it is draining. If you are using a ball valve to turn back the flow you will shorten the life of your pump and use more electricity than you need to. So if your overflow is draining 600 GPH it doesn't make sense to get a pump that is pumping more than 600 GPH after you have calculated your head loss.
Since your return is pumping water up against gravity and will have to travel through vinyl tubing or PVC to get back into you will need to calculate your head loss, which you can via this calculator:
http://reefcentral.com/calc/hlc.php
For example, I calculated the head loss using that calculator given the following figures:
Using the following input parameters
Friction Factor = .24
Vertical Length = 4
Horizontal Length = 1
Pipe Diameter = 1
Flow Rate = 600
90° Elbows = 4
45° Elbows = 0
It gave me these results:
Head losses are calculated as
Frictional head loss is 6 foot head or 2.59 psi.
Elevation head loss is 4 foot head or 1.73 psi.
Total head loss is 10 foot head or 4.32 psi.
Take the total head loss in feet and find a corresponding chart for whichever pump you are planning on using. Here is the Mag Chart:
http://www.marinedepot.com/ps_AquariumPage~PageAlias~powerheads_pumps_danner_mag_drive_supreme.html?CartId=
So, using the chart above you find the figure on the left that notes 600 gph at 10'. In our case that would mean a Mag 12 would be able to push 600 gph at 10'. Since our drain is rated at 600 GPH a Mag 12 is perfect.
You will probably be okay with a Mag 7 depending on how you do your plumbing. It will be either that or a 9.5. There is no Mag 8; however, there are many different brands of pumps out there to use. Eheim is usually considered the best--most quiet, efficient, cost-effective, and reliable, but carries a large price tag. Mags are pretty middle of the road pumps that tend to give off a lot of heat but are fairly reliable. These are just a couple of internal pumps. You also have the option to use an external pump that is plumbed into your sump via a bulkhead on the side. So, as you can see there are a myriad of options.
Plumbing-wise, the easiest is using braided vinyl tubing. I prefer using schedule 40 PVC to "hard" plumb my system. It really a matter of preference. I feel hard plumbing is "safer" but certainly not as convenient as using vinyl tubing.
A lot of people include their sump and refugium in the same tank, which is divided by baffles. Refer to Marc's website for a better explanation; however, you can also use two separate tanks--one "sump" for equipment, skimmer, etc., and the other purely a refugium. It's really up to you.
I would also think about seeding your sand with a cup of live sand from someone's established tank. It will jump start your system and help you introduce a nice variety of microfauna and bacteria. Most members are happy to donate a cup for free if you can find someone in your area or go to a meeting.
I would consider getting a better skimmer if your budget allows. It is possibly the most important piece of equipment you will use. There are a ton of good brands and prices across the board. If you're buying new expect to spend around $250-$300 minimum for a decent skimmer. If you're buying used you can find good ones for half that price on the sell/trade boards here or on reefcentral. Try and purchase a skimmer that is rated for double the volume of your system. So if you go with a 30 gallon sump, try and find a skimmer that is rated for around 200 gallons.
If you are DIY minded, the DIY forum on reefcentral and here on DFWMAS has some plans that allow you to upgrade your coralife skimmer for much less money; however, it will take time to dial it in once modified and may not be worth the hassel. It is another option for you though.
Since we're in Texas, depending on where your tank is located in your house and what temp you keep it at, I would think a couple of 100 watt heaters would be fine. 200 total watts would be more than enough to keep your system at or around 80 degrees. I use a 50 watt heater for my 40gallon + 20 sump/refugium and it doesn't get below 78 in the winter. I would suggest using some kind of controller in conjunction with your heater, however, because even the good brands might sometimes get stuck on and cook your tank.
Finally IMO you should invest in a Ground Fault Circuit Interrupter ASAP. They are wired to automatically cut power to whatever is plugged into them if water comes in contact with the powerstrip, etc. You can get one from Home Depot or Lowes for around $20-25 if there is not one installed in your wall or you don't know how/have the desire to wire it yourself. They are three pronged and plug into a regular outlet.
Your lights should work fine as long as you're not trying to keep most SPS except for a few species of montipora that do okay under Power Compacts. If you're like me you will probably want to upgrade to metal halides or T-5s, in order to allow you to keep colorful SPS or Tridacnid clams, for example. In general, most photosynthetic corals will grow faster and produce better coloration under more powerful lighting in conjunction with adequate flow.
I hope this helps and feel free to pose more questions as they come up.
