Anarchemitis said:
Now would it be logical for me to be contemplating the GeForce 9 series?
Depends on the depth of your pockets. Here's a link to an article at Tom's Hardware on the best video cards for the money as of June '08.
http://www.tomshardware.com/reviews/graphics-cards,1942.html
Series numbers can be confusing, as the first number is the generation and says nothing about the power of the GPU. The 9800 GX2 is the king at the moment; it has two 8800 GTS GPUs more or less. Below that are the 8800 Ultra, 8800 GTX, and 9800 GTX, in that order. The ATI 3870 X2 is more or less on this level, also having two complete GPUs on one card. Below that you find the 8800 GTS 512MB and then the 8800 GT 512MB. The next level is the 8800 GTS 640MB, then the 9600 GT and 3870, then the HD2600, and so on. There's a nice chart on the last page of the article. Tom's does this article every month. There's also a Desktop VGA Chart on Tom's Hardware here:
http://www.tomshardware.com/charts/graphics-cards/3dmark06-v1-0-2-hdr-sm3-0-score,538.html
This chart allows you to choose one of several benchmarks and see how different cards perform.
Another way is to look for benchmarks for the games you want to play; some web sites regularly do these for GPUs and sometimes CPUs (not as important) for popular games.
One thing to keep in mind when looking for nVidia GPUs in these articles is that different graphics cards can be clocked at different speeds than the stock reference design. A factory overclocked 8800 GTS 512MB might be faster than a stock 9800 GTX, even though it's slower on the charts.
"DXDIAG.EXE" also works in Vista, but not as well. (If you've had Vista long you probably expected that last part.) Here's a wikipedia link:
http://en.wikipedia.org/wiki/DxDiag
I'd go for a factory overclocked 8800 GTS 512MB or 8800 GT 512MB myself unless you have money to burn or couldn't afford these. In the first case the 9800 GX2 rules; in the second case the 9600 GT (or the 3870 512MB if you prefer AMD/ATI) is the lowest I'd go for a gaming PC, and both deliver good bang for the buck. Graphics are the most important part of a gaming PC with the possible exception of sufficient RAM, so don't scrimp below what you can afford if gaming beyond WoW is a priority.
You may also need to upgrade your power supply for a new video card. That information should be available on the HP web site. Good luck.