best card for these specs?

7600 would fit nicely.

If you're willing to upgrade the PSU, then X1950Pro or 7900GS would be nicer.

would that be a GT or a GS?

My experience with HP's is limited, but they seem to be good, but simple computers. Overclocking and upgradability is minimal at best. Like Prozac said, get a bigger psu. That 300wt is minimal and it won't take on any extra load should you want to change the video card or much or anything else.

You would definitely want the 7600 GT. As stated before, if you are willing to upgrade the PSU go 7900 GT/GS or x1900xt.