Need help with video card upgrade

I want to upgrade the graphics card in this computer but I don't know exactly which ones will work in it. I will be using this computer for gaming primarily. I want to be able to play games like COD4 and of course any newer games that are released in the near future that catch my attention! I do not know what kind of power supply or slots this computer has because I have not received it yet, but would like to have a new graphics card in hand when it arrives if possible. I have a budget of up to 350.00 for card and PSU(if needed).

Specs:
Studio XPS 435 Desktop: Intel Core i7-920 Processor (8MB L3 Cache, 1MB L2 Cache, 2.66GHz)
640 GB SATA Hard Drive (7200 RPM)
256MB ATI Radeon HD 3450 supporting HDMI
6 GB Tri-Channel DDR3 SDRAM 1066MHz (6 DIMMs)
Windows Vista Home Premium 64-bit

I also asked in the Dell forum if this is a computer that I will be happy with for a while or if I would be happier with the performance of a Q9x00 chip since I primarily just game. Any thoughts on this? I grabbed this for 679.00 but have been having second thoughts on whether I should have just went for a quad.

Ok first off the i7 IS a quad core chip. It is actually a Hyperthreading Quad core which so windows will see it as 8 cores. Trust me you did best by getting the i7, it rocks the hot totty.

Looking at the XPS Studio PC's i see they package either a 3450 or 4850 card with these desktops. I am assuming since you want to upgrade you are getting the 3450 card. The Specs state they have a 360W power supply. I am ALSO assuming this is across their entire line of XPS Studio PC's. Assuming this, I would say it is safe to install a 4850 card in this machine since they already offer this as an option. 360W is not alot to work with but obviously they have tested this with a 4850 already and sell this option.

I would get a factory clocked (non OC) 4850 card. It is your safest bet. It will have NO issues running COD4. I built a machine with a 4670 and it ran COD4 maxed, but no AA smooth as butter. A 4850 will do you well. They are about $150. I wouldn't try anything more unless you upgrade your PSU.

My real suggestion would had been NOT to get a studio PC but rather one of their XPS standard desktop. They come with a 375 or 425W PSU. Or at least get the PC with the 4850 option already, at least Dell would follow through with a warranty.

Cancel and reorder with the HD4850.
If I was buying a Dell, which I never will (it`s much more fun to build your own ) I`d go for the Studio, despite its other shortcomings, because it has the i7 and 6Gb of memory. As a core system it`s going to be good for a long while to come.

Thanks guys for all the information, it helped more than you could imagine! I have decided to go ahead with a HD4850 and see how it goes from there. If I'm not happy with it, I will definately be picking up the ugly PSU and HD4850x2 later on.