Welcome to the Forum!
You can view the differences in most of the Ibanez 7-string models here:
http://www.jemsite.com/axes/htm_features/spec_ib7.htm
Basically, the RG7321 is a Korean-made
fixed bridge guitar. The RG7420 is a Japanese-made guitar with the LoTRS7 tremolo. Ideally, you should be able to find a used RG7620 for a decent price. They're killer guitars and are top-notch in terms of quality and materials. LoPro7 trem,
DiMarzio pickups, etc. The seem to be going for well under $500US used (some as low as $300-350 if you don't mind some finish wear).