I've been running VMWare Fusion on a white 13" MacBook and mid-2009 MacBook Pro with Win XP for several years, and more recently Win 7 64-bit.
I found that with 2 GB of RAM, both OS X and Windows ran noticeably slower than with OS X alone. Upgrading my RAM to 4 GB, and then 8 GB, made dramatic improvements in speed - everything was more responsive.
However, upgrading to an SSD (Intel X25-M G2) made the most dramatic improvement of all. Whereas doing anything disk intensive in Windows using a standard HDD would bog down the whole system, with the SSD I can install Windows patches and apps in the background (with frequent automatic rebooting) with no noticeable impact on the rest of the laptop (OS X apps).