According to cnet, Nvidia recommends a minimum 400W PSU for a sytem using a single 8800 GTS.
To its advantage, the 8800 GTS card requires less power than the GTX. Nvidia recommends at least a 450-watt power supply for a single GTX card and 400 watts for the GTS.
Link
I should appologise for suggesting a PSU with less than 400W in my last post, but 500+ is overkill. The SeaSonic 430W PSU goes for $95 at newegg, which would save you $20-30 over more powerful models. Going for more power now would, however, let you consider playing around with more hardware down the road, such as big RAID setups. The 430W SeaSonic is what I have on order from directcanada, and I paid just CAN$79.82, which today translates to US$69.19. This is why I was suprised when you said SeaSonic was out of your price range. Maybe they're just cheaper in Canada.
EDIT: Get an impression of system power consumption from AnandTech:
Link
Also consider that the system components used in Anand's review are relatively power-hungry. For example:
CPU: Intel X6800
NB: nVidia 680i
The article might also give you an idea of what kind of monitor you should have to take advantage of the power of the 8800-series cards. They're running a display with a native resolution of 2560x1600.
I've seen different numbers from different places (Though I tend to believe Anandtech's numbers over anyone elses).
For example
here they show a system with a similar config consuming 315 Watts of power.
Again, the numbers you're looking at are averages, and they don't account for spikes. 400Watts might be fine, but I wouldn't risk it personally. You're going to want some headroom when it comes to your PSU.
Either way I think we're splitting hairs. It's not worth risking a US$400+ video card with a $50 PSU - to save $20 or $30.
Another good point (made by others) is that the GTS is overkill for 1280x1024. I run 1680x1050 on my 320MB GTS and I've yet to encounter a game that it can't handle well (Including Oblivion @ full settings). You'd still do very well with the 1950 Pro or XT. I'm just mentioning this because you said money was an issue.