Yeah, you don't require Lion. You just need a Mac that can run Snow Leopard (intel Mac, C2D would be better then one of the original intel macs that weren't C2D) - more ram is better (mostly just for performance) so 4Gig (or more) would be good. XCode 4.0.2 runs on Snow Leopard and builds apps for the latest release version of iOS. XCode 4.1 is the lion version - it's no different from 4.0.2 aside from supporting Lion specific features (resume, full screen, etc.) Even the next release of iOS (5 which is in beta now) has a version of XCode for Snow Leopard, so you'll be fine.
If you get a Mac with a C2D processor you can update to lion at any time should you choose to do so (at that point then you will definitely want 4G+ of ram).
I've developed on a variety of machines - the one I'm on now is an older iMac with Snow Leopard with only 2 gig of ram and I have XCode 4.0.2 on it, my Mac Pro at home w/ 8 gig has snow leopard with 4.0.2 on it and my Air 11" has Lion with XCode 4.1.x on it and develop on that all for the current version of iOS.