Are you not comfortable with building your own system?
I would not recommend the x300 series video card, they just came out with the x550 series (card is not ATI, but chipset is) that will out perform the x300 series and sells for about the same price.
Nothing wrong with getting a dell unless you intend on doing any modification of your system
If you like Intel the new dual cores are cheap..
If you want raw gaming power, get yourself an AMD 64bit .. the venice cores are very nice and greatly clockable
Also if you are looking at a piece meal build... what you can do is get a mobo processor now... and your current rig you can dissassemble for a little while until you complete the new rig.
What I mean is... use RAM Power supply Case... hard drives from your current rig in the new one. As you upgrade the new one, repair your old(present) rig with the components you borrowed.
For greater gaming performance you are going to want SATA hard drives as well.
Good Luck whatever you decide ^*^