PSu upgrade required......but if u are willing to settle for a 3850 or 9600....then it might be ok....i run a 9600GT on 420W PSU....but its borderline....3850 would be a much safer option provided u keep ur current PSU
The 8800GT uses about 78watts at load http://www.atomicmpc.com.au/forums.asp?s=2&c=7&t=9354
That system including a 8800GT is still well under 300 watts. http://extreme.outervision.com/PSUEngine
A quality 400-450w power supply would be fine. The biggest concern is that Aspire power supplies don't have a great reputation.
If it was my system, i'd install the graphic card and run it, but i'd also keep an eye out for good deals on a quality power supply and replace it when i could.