8GB is min for ML and W7. Start there, upgrade if you need to (easy to do yourself, and be sure to buy the Mac with minimum RAM to save $$$).
I'd go big on the internal drive, and also get an external for time machine (or some other method of your choice) backups. My system has a 500 GB internal, and I'm using a FW800 Guardian Maximus 2x2TB (hardware mirroring).
All that said, the new iMac *should* be coming along someday. Even if it's just a speed bump, I'd anticipate USB3, Thunderbolt, and Ivy Bridge. Many are hoping for and SSD+HD option (me included). Worth the wait unless you really need it now (or have the wife's "OK" RIGHT NOW, and don't want to lose the opportunity LOL).