" I would perfer NVIDIA over ATI, but I'm open to any advise."
I don't use nvidia cards but I can offer some advice if you do go with ATI .
The "pixel shader" problem is a lot of the newer games require "Pixel shader 3 [or 3.0] .
In the ATI cards any one from the X1300 series up will have pixel shader 3 .
Avoid the X1800 & X1900 cards as they require a LOT of power to run & the stock Dell power supply probably won't be sufficient.
For a "low budget" upgrade a X1300 Pro or X1300 XT can be found in the "low profile" option .
Look for at least 256MB of memory & at least a 128 bit memory interface.
I have put X1300 Pro's in several computers.
While not the "fastest" video card they will play most games quite well .
Of course if you have the $ to spend get the best you can afford !
Here is a list that may be of some help as to what is "fast" http://whirlpool.net.au/wiki/?tag=rmp_vds
Avoid the video cards that require a 4 or 6 pin power supply connection.
["Big" connectors like a IDE hard drive ]
Some have a small 4 pin power connection that is the same as a floppy drive [the ATI X1300 series has it] most power supplies will power those with no problem.
OCZ StealthXstream 700W,Gigabyte GA-EP45-UD3R , E8500, Arctic Freezer Pro 7, 3GB G.Skill PC8500,Gigabyte Radeon HD 4850 OC [1GB ], Seagate 250GB SATA II X2 in RAID 0, Samsung SATA DVD burner.