You should buy new, so there are only 3 versions:
1. Arcade, no hard drive, no cheap HD upgrade any longe, $199 list.
2. Pro, now has 60 GB space instead of 20 GB, $299 list.
3. Elite, still only 120 GB and still costs $100 extra at $399.
Pro has about 45 GB free after MS grabs some GBs for caching.
You should be able to install at least 5 games to the HD at once with room left over for saves & DLC.
So, Pro makes the most sense for most people, unless you want to keep more than 5 games installed to the HD at once.