You'll need to post a bit more information about the rig you're proposing. CPU and video card type, for a start. We have quite a few people around here who are good at evaluating that sort of thing but they need more details than you have currently provided.

You might also want to mention what game you're planning to multi-box....

Between now and your birthday, you'll certainly find the following links useful in terms of actual information teaching you how to go about the process of multiboxing (both on a single machine and multiple machines):

Guides

Software tools forum

Forum search facility

Better, but off-site search facility

Wiki

Video - how to set up your first group in WoW

Have fun!