There's no way around it. 64bit is the way to go, it's where we are headed.
Most people who''ve purchased a computer in the last 5 years probably already have a 64bit processor but are still running a 32bit OS. Windows 7 x64 is a great OS and you can get the upgrade version for less then $100. if you're already going to throw down a lot of money for CS5, mind as well put down a little extra for Windows 7 and some more RAM (if you're gona run 64bit, make sure you have more then 4gb).