Over at Tom's Hardware, they have done a neat article on running Vista on a budget PC. Can it? and do you want to? Here's an excerpt:

Windows Vista turns out to be somewhat arbitrary in the way it assesses system performance. While there are significant visual differences between the AMD Sempron 3400+ and the Celeron D 352 systems when it comes to 3D capabilities, Microsoft's Vista Experience Index doesn't do justice to the AMD system. It is true that the AMD system cannot display transparency effects for AeroGlass due to the motherboard and chipset. But Vista's performance index only rates the PC with an overall system Experience Index of 1.0, while the system powered by the Intel 945G chipset and a Celeron D 352 reaches 3.0 points. The Celeron PC rates such a relatively high score because the Intel chipset has a more advanced 3D engine, but its 3D performance is still inadequate for gaming. The graphics score, which is the dominate score for the overall index, thus creates a rather misleading picture. The two systems are based on cheap integrated graphics that are not intended for gaming any way. The index hence doesn't reflect our everyday experience, and we recommend not basing a buying decision on it at this time (unless you're looking for the highest score).

At the same time, we found something really valuable in the Microsoft toolbox: the Windows Vista Upgrade Advisor. Although it will always recommend a new graphics card (unless you have a really new, discreet graphics card), it is a very helpful tool to assess your components and software in order to decide whether or not it makes sense to switch to Windows Vista today. It can do a great job at warning you against upgrading if you're running hardware or software that Vista doesn't support, and it gives you a recommendation for a version that matches your hardware.