He'll get about 33-40% of the cards raw performance.
However, things like AA, HDR and ansiotropy are all handled entirely on the graphics card. So he'll be able to crank those up without dinging his frame rate. Downside is he'll never go above the modest frame rates.
The severity of the bottleneck will also vary dramatically from game to game. Keep sound quality to low and make sure any physics is set to low or turned off. Also make sure his power supply can handle it. Older systems especially brand name systems, will frequently lack the amperage on the 12v rail to handle the more recent cards.
eta: just noticed the very low resolution, it'll be severely bottlenecked that low. Probably score around 4500 +- 500 points in 3d mark '06.