I am guessing that this is your system, but with a Quad core as you said in your post.
Aspire M3641
Your system will fit a 9800GT
But unforutnatly the minimum power supply you need for that card is 400W.
I would also suggets looking at ATI cards, this is because you get alot more for your money.
The reason for this is as follows:
The ATI contender for the Nvidia 9800GT, is the ATI Radeon HD4850 [the card that I personally have]
Both cards are around the same price, but the ATI gives out more graphical power, as shown by Nvidia's own tool to check how fast your system will be with the chosen card if it was in your machine, and it showed that if I swapped my HD4850 for a 9800GT, it would actually lower the performance.
So overall, I would chosoe the ATI HD4850 which can be found for about £100 cheapest, about the same price as the 9800GT, and they both need around 450W minimum power supply.
Hope this helps and wasn't too confusing.