There's a single file on the disc that defines what "version" of Windows is it. If you look inside the sources folder on a disc, you'll see a file named "ei.cfg". This defines what version of Windows 7 the disc is for. All you need to do is create a new disc that has the ei.cfg file removed, and you'll be asked during the install which edition of Windows 7 to install. Personally, I prefer a USB key that has the ei.cfg file removed.
Note that you can't combine the 32 and 64-bit versions onto a single disc using the "simple" methods. However, with a little googling, you'll find forum posts from people that have combined both on single discs/USB keys, or have even combined Windows 7 with Server 2008 R2.
Intel i7 860, Asus P7P55D Pro, 4x2GB Corsair XMS3 1600 (CMX4GX3M2A1600C9), EVGA GTX 560 Ti Superclocked
Seagate 7200.7 160GB, WD Caviar Black 640GB, WD Caviar Green 1TB, WD Caviar Green 2TB
Dell 2408WFP and Dell 2407WFP-HC for dual-24" goodness