Graphics Card question

Hello everyone. I want to upgrade my desktop, and I want SL to run better too. I really dont know too much about this stuff so I was hoping someone could chime in with some feedback. I dont know all my computers specs, but it is an HP that is about 3 or 4 years old. It has a Pentium D processor and its 3Ghz.
It has 2 Gb of DDR2 RAM and currently it has a ATI Radeon 300 128mb video card. My boyfriend and I were in Best Buy the other day, and he says that I should get a new video card, but that I would likely need to get a new power supply too?
We were looking at cards, and I noticed that a 1Gb Nvidia 9800GT was cheaper than a 512mb 9800GTX. What is the difference? If I went with either of these, am I going to have to do alot of other stuff too? Are there better choices then the Nvidia cards?
Thanks so much for your help!

Beware the GTX cards. I have a friend who tried installing one recently but had to return it because it would not fit. These cards are very long - they will go about an inch or two beyond the edge of your motherboard..

edit: dunno nothing about ATI cards so I've got no idea how it'll compare with what you have now.

The biggest problem you face will a pre-built machine, especially one that old, is the power supply. You will need to find out its wattage before exploring your options. You probably won't be able to upgrade the power supply as the case is non-standard most likely.

Given the age of the machine you are best off building a new one from scratch. While you could get better Second Life performance from any newer card that will run in your current machine, you are kind of at an upgrade impasse with that technology and might be better off planning a new computer.

I did a little googling and give or take it seems that the GTX will probably perform about 15% faster than a GT but those results could vary depending on your system. I'm not sure how much the extra VRAM in the GT would help you but I can say that just recently I upgraded my sl machine with a 512MB 9800GTX and it gave me a significant improvement over the 256mb 8800GTS I replaced (probably a 25-30% improvement in frame rate). The card is very large as someone else mentioned but it fit fine in my mid-tower case.

I think the real question is what your disk cage looks like. The case in question, the one where it didn't fit, had a (rivited-in) disk cage that went from just under where the optical drive lives all the way to the bottom of the case. If that wasn't there, the card would have fit.

It's easy enough to check if your case has enough leg room for one of these cards - don't assume that a mid (or even full) tower will be big enough.

I just upgraded my card.
After a lot of research I decided on the 9800 GT which is cooler and smaller than the 8900 and got super reviews, basically the best in its price range.
It outperforms the GTX by all accounts.

I did have to also upgrade my power supply which I did to 650. I did NOT have to get a new tower to accommodate that.

Although that card, like the 8900, tends to run hot, it's very steady. It runs at 74 under normal conditions but doesn't go over 78 while in SL.

LOVE it

B>