I take it the Adrenalin EA Boot Fix didn't work... Sorry about that.
1. You don't download the software for the HD, that would be piracy, however do a search for HD Loader on Google. Hopefully someone is still selling the original HD Loader Disc. DON'T BUY THE HD ADVANCE. It's just a clone and a ripoff of the original. A friend of mine bought an authentic one about a week ago so hopefully they are still available.
2. You can go higher than 120 gigs if you have a special patched version of the HD Loader. These are not for sale, and you need a modchip to boot the CD-R. You can fit approx. 40 games on a 120 gig depending on the size of the games you install. You can always have 2 Harddrives.
3. As far as online gaming I am not sure since I don't play online...I want to say yes you can but I may and probably am wrong. Hopefully someone else will either confirm or correct me.
Hope this helps.