Which of these GPUs are good for my Dell XPS 420? Looked at the store

9800gt will max out wow at 1680 by 1050. 4670 is another terrific card. I'd buy the 4670 as it has 55nm manufacturing fabrication and will run cooler and use less power. ATI has been making great cards since the 4000 series was introduced and I'd still stick with them if i were to buy a new card. I have been using a 4850 for a while and i love it. check this chart out http://mark.zoomcities.com/images/gfx/GFXpowerchartbyid....
your other components use roughly 170watts. the 4670 uses around 50 watts when maxed out, the 9800gt draws 80 when at maximum throughput. either way, your power supply is more than adequate.

9800GT is the best performer next comes HD4670. Your power supply will run either of them so my vote goes to the 9800GT.

Thanks everyone. I thought the more memory the better for GPUs? Why is the 512MB better than the 1GB ?

The difference between those cards lies in the GPU it self not the memory. Many lesser cards come with 1gb of memory although the GPU is not powerful enough to utilize it. Here is a good read about ranking of GPU's http://www.tomshardware.com/reviews/geforce-310-5970,24...

Cool thanks. How much better performance will the two top cards give me over the 8800GTS I have in the machine now?

Which 8800gts do you have? There are 3 possibilities. If you have the best one which is 512mb going to 9800GT would be a down grade.

I am sorry it is a 8600 GTS.

They would be multiple times faster than a 8600GTS

Cool thank you

One more thing, what would be the best video card that I can use with my Q6600 Core 2 Quad 2.40 system? What cards am I limited to get good performance?

If you overclock the CPU and replace the power supply the sky is the limit.

I am unsure about pinning correctly and what if I only upgrade the power supply and change to PC6400 ram instead of the PC5300 ram that I have?

What if I upgrade to a faster processor like a Q8400 or Q9450?