Alienware systems are stupidly expensive.
I recommend going to ibuypower.com and getting an Athlon based system. You get the same processing ability at a lower cost. I use an Athlon XP 3200+, and it's screaming.
As for Video Cards, the nvidia Ti series is a thing of the past.
If you are looking for a decent medium-high range card, I'd recommend a Radeon 9800XT. It's the renewal phase of the 9800 (one step up from pro) and is in and of itself a very snazzy card. The x800 is out now, and is much more powerful, but also runs upwards of 500 dollars. If you HAVE that.. go for it, but you really won't find many games really writing with that card in mind. For ATI cards I recommend ATI brand, or Sapphire. Stay away from shady brands that seem to only sell their stuff on the deeply discounted sites, they really aren't worth it.
If you really like nVIDIA, Gainward is your best bet for cards. They are an *awesome* company, using higher quality DDRAM and higher quality fans. You can also get a FX5950 with a water cooling system that just *screams*
If you want something more of a bargain, grab yourself a 9600XT from newegg.