Well , here is a first hand experience :
http://www.shacknews.com/featuredarticle.x?id=1090
The author played through some chapters in BioShock , the menu was responsive , but the gameplay suffered from noticeable lags during fast mouse turns , and above all the picture quality was terrible ,well not that terrible , but not what he expected , he said that the signs of compression really marred the experience .
Besides , What's wrong with upgrading your PC every now and then ? a careful planner could only spend 800$ on an average PC that lasts for 3 Years (one with a decent C2D and an HD 4850 or 9800GTX+ for example , spending more will extend this period even further !