Yup...pretty much what everybody else is saying. You're money's better spent on a higher speed single CPU system for what you're thinking to do. Basically the OS, application, and the process the application is executing..all have to be multi-threaded to reap big advantages.
Five years ago I had a dual-700 server I converted to a gaming machine (GeForce 2, RAID 0, lotsa fun!). For gaming it was the same speed as a single 700MHz machine (with the same video card). It did have the advantage of still running nearly the same speed if I multiple applications going on it. For example I could have some programs running and still game at full speed while a single CPU system couldn't do that. It did take a little tweaking though.
As has been mentioned, Win98/ME/XP Home doesn't do SMP at all and only sees one CPU. Win2k/XP Pro do have SMP capability, but it isn't all that great. You can set an "affinity" for an application to favor a certain CPU, but it's a little bit of a hassle.
SMP Seesaw is pretty good for separating apps to different CPUs for you. It still doesn't speed up any games or anything. It will still be the same speed as a single CPU 2.4. But you could be decoding a video file on one CPU and play a game on the other CPU and not slow down like a single CPU system would.
Once apps and games are written to be multi-threaded, multi-CPU (and multi-core) systems will wipe the floor with single CPU systems.