some motherboard/ graphics advice please

hello all
ive finally decided to buy a graphics card to replace the on board chip that came with my pc but i was wondering if i get say, a new nvidia card will it automatically override the nvidia onboard chip or do i have to fiddle withe the bios?
im running:
amd athlon xp 2600+ (2.13ghz)
2gb DDR ram
Asus A7N8X-LA motherboard

any help would be great....
also im happy to spend around 300$ (australian) on a new graphics card if anyone wants to suggest a particular model!

thanks,
pete Mcpete

 

thx rahul,

neglected to mention, the sticker on my psu says its 250W max. am i going to have to get a new psu to run a decent nvidia card?

 

Most likely.

 

250 watts? That's a 100% certainty!

 

yeh hehe ul want about 400w to run a top notch card....just dont try to run it in SLI