First off, I disagree with others saying SSD... that will only improve boot time and apps running off HDD only. Also, IMHO they are still overpriced! Also you may experience "issues" with an HP OEM PC and an SSD.
You also didnt say if your Windows 7 is 32 or 64 bit in specs... Also, a very important item... how much can you afford or do you want to spend?
And... what is your main usage for this PC??? In otherwords, where would you like to see the performance improved???
Your current mobo/cpu can accept a max of 8GB ram but you need to be running 64 bit Windows 7 to take advantage of anything over 4GB. See HP Specs here: http://h10010.www1.hp.com/wwpc/us/en...8-3459241.html
<--- BTW these specs state your video card is a 8400 GF which only has 256MB of video memory (an 8400 GS has 512MB)!
Just basic answers here... (assuming that you dont want to upgrade Mobo/CPU)
If you are a gamer and want to improve performance in games, then yes a video card will do that very nicely! By todays standards your video card is very old, feeble, and weak!
If not a gamer and you run a 64 bit version of Windows 7, then an upgrade to 6 or 8 GB of RAM for all around performance improvement.
Both options I stated are good "bang for the buck" upgrades! IMHO!!!