Consider few things before u decide on graphics card, i am a user of onboard graphics , albeit a much older version.
1.Do u want to play games at max graphics settings, like enable shadow, high texture, unique details etc. If yes then go for graphics card.
2.Do u want to play high definition games which will come out in future. Most of onboard cards support its own generation of games pretty decently. But the problem usually appears after a year or two when u will not be able to play any new games at all.
3.The price of cards stabilise / stop reducing after a certain point. If u want to go for directx9 card, which in my opinion is not required as of now, then u can well get a graphics card now itself, coz most of those prices will not reduce much any more. However if u want to go for directx 10 card, then u can very well wait for atleast a year for decent prices. Second option is better as your onboard card will suffice for a year.
Also consider getting 2 gb of ram now itself, coz often ram upgrades are painful if u dont get the matching pairs. More so if u are not going for separate graphics card as u will need more memory in that case.
