I've seen 700 MB PSX discs compressed into under 70 MB

Yeah, it varies from game to game too
Tweaking how the compression algorithm works too can also produce different results. For example with rar, you can select how strongly you want it compressed (traded off with time) in WinRAR. You can select Solid Compression too to allow it to compress across files rather than the conventional way of compressing files individually and putting them together in an archive - this has disadvantages when decompressing though if you want a single file, as you'll then have to scan the entire archive from the start. You also can manually increase the amount of memory it uses to allow it to match a wider range of patterns and so compress more. Don't give it too much memory though - if the PC can't handle it, you're gonna see a really nasty increase in time as the computer starts using swap file space instead of physical memory. Shame there's an upper limit of memory you can assign though (maybe hard coded into Winrar rather than being algorithm dependant, I don't know) - I'd like to give it 900 MB or so combined with Solid compression to allow compressing across several ISOs that contain some areas of similar content so they compress even more
