http://www.alibaba.com/catalog/10911...esktop_PC.html Also I have upgraded it to 2 gigs of ram.
I have tried before unsuccesfully and after a few years of owning it its starting to show its age in games such as Vanguard online, Battlefield 2, and Lord of the rings Online.

I was wondering if it is possible to upgrade this computer and if so what would be the best video card to buy for it.
Also since one of my parents is doing the upgradeing this time could anyone give me a link to some step by step instructions on how to generally replace a video card.

According to http://www.bizrate.com/desktopcomput...formation.html this pc has PCI Express card so you can buy any PCI Express card. However before you do so, you should check your power supply. New graphics cards needs PSU above 400Watt. They are power hungry...
As to choosing card it is up to you, what brand you preffer (nVidia or ATI) and how much you can/want to splash on it. So provide price limit and people around will give plenty of ideas

The max im willing to spend is about 200. As for companys im not sure which i prefer.

Also how do i check my max power supply?

You need to specify currency

All power supplies have label with this info, the label can be found either next to socket for cable or you may need to open case and look inside

$200 Us

hmm, that can be tricky...
I paid £250 year ago for Power Colour X1800 XT with 512MB but since then prices changed. Quick search on amazon come with this card ATI Radeon X1950 Pro HD PCI Express 256MB which is good value for money
also other like XFX GEFORCE 8600GTS 256MB DDR3
or try to run price search for any of this Power Color cards

When i looked up those two cards on newegg they go decent reviews but most say that although its a good deal it wont be able to run games like oblivion oh high settings what kinda card will that take?

you have only two option:
1. get extra money so you can buy top gaming graphic card
2. buy what you can buy right now

my x1800 has no problem to run bf2, cod2, nfs:most wanted on top settings, where ATI Radeon X1950 will do extra fps on these games

Im also upgrading my computer my current motherboard only supports agp and im upgrading it to an Sli PCI-E motherboard and all i know is that pci-e= 4000mb/s and agp is 2100mb/s but whats that in gaming language, how would that effect image quality and frames per second?

eg. i have a computer with a 7300GT agp 8x and i get say 30 fps, what frames would i get with the same card and rig only difference is the video card is pci-e?.

Difficult to say, but defenetly more. I had Sapphire x 1800 which did 60-70fps on cod2 but because it broke down I got replacement in Power Color X1800XT which does 85-100 fps on maps where is lot of dust