Computer question, video cards?

ello, so im pretty stupid when it comes to computers, i just play it alot cause ima huge gamer, but lately i been looking at games an i go to systemrequirementlabs an it always says my video card fails for recommended, so it says i have You Have: GeForce 7100 / nForce 630i and well i take it that its not good lol, can anyone tell me a good video card that aint to high cause i aint rich lol, so a good video card better then wat i got an not to high, like im kinda bored wit wow after playin for years so i been lookin a a game called vindictus an it says Recommended: NVIDIA GeForce 7xxx series or equivalent so sumthin around there? also real quick my computer is a HP pavilion slimeline an i been told that sumthin wit the mother board or watever the hell thats inside there sumtihn is too small an i cant add a video card without replacin alot of sh*t, is that true? idk as said before im not smart wit computers