You're not exactly giving us anything to go on. How much are you spending? Do you have any parts already? When are you planning to buy/build? What country are you in (websites and prices differ drastically between countries)? Basically, you need to follow the guidelines from the link in my signature.
What I can tell you is that unless you're doing standard tasks (you aren't) and/or have a really low budget ($400) you're better off not dealing with Dell (or HP or the like). Also, it's better to avoid custom builders (like CyberPower, IBuyPower, Digital Storm, etc.) altogether. The reasons for this are simple.
One, they'll use very low quality parts when it doesn't matter. The PSU, motherboard, RAM and HDD are going to be old and basically the worst quality you can find. That's how they save costs. This is especially true with Dell and the other prebuilt vendors like them. Sometimes, they'll even use proprietary parts that will only work with parts you get from that company. That'll ruin your upgrade path.
Two, you'll overpay. Once you start talking about spending $500ish (or more), you'll be paying a lot for the privelage of buying a complete build. You'll be able to find better performing, higher quality parts on your own than having htem selected for you.
Three, you're buying poor build quality. This is mainly true of the custom builders. They'll shoddily throw everything in the case, and do just enough to make sure it works. The computer might arrive damaged or improperly built. Instead, if you build it yourself, you can make sure everything is put together carefully and correctly.
I'm going to take a stab at a build you should consider (assuming the budget is high enough). Keep in mind that Intel's new CPUs are due out very soon, so this will change immediately on their release.
Mobo: Gigabyte GA-X58A-UD3R
RAM: Any non-OCZ 3x2 GB kit of DDR3 1600 mhz CAS Latency 7 sticks
HDD: Samsung Spinpoint F3 1 TB
GPU: GTX 460 (at the least) or GTX 570 or GTX 580
PSU: A 650W (for the 460) or 750W (for the 570/580) from Corsair, SeaSonic, Silverstone, XFX or Antec
Case: Largely depends on your tastes. Some good options: HAF 912, Coolermater 690, Antec 900 (or 902), HAF 922, or anything from Lian Li.
Optical: Cheapest SATA DVD burner you can find.
OS: Windows 7 Home Premium 64-bit