Yarlen, since you recomended this site, do you know the voltage of the standard power supply they give there?
You don't need to worry about those nitty-gritty details. You just need to worry about overall wattage in a pre-built system like this. 
Few comments:
1) You're going to need a better power supply than the default for this system (I'm looking at the second one). I would strongly recommend at least 550-600 watts. Video cards, especially under heavy 3D conditions, suck down a lot of juice. You don't want your system suddenly blue-screening or powering-off because your supply wasn't up to the task. It may cost more, but at least with ibuypower you can change it. With Dells you can't, and if our systems in the office are any indication, they tend to go with the bare minimum power supply. If you can't swing a better power supply, then I suggest downgrading the video card a bit.
2) Since it's only $20 more, I would upgrade the CPU to the E6420.
3) Hook yourself up with the free games/t-shirts they offer (what the heck, you're paying for it).
4) You can go with the certified heat sink/fan. No reason to upgrade this.
5) Hard drive - if you need to bring the cost down, get the 250GB one. You can always add another one in after-market (very easy, lots of step-by-step instructions online) for little money.
6) There's no point in getting 2 DVD/CD drives. Just get the 1 DVD/CD RW and save the money.
Those are my recommendations.
