I know the 661 will take a 16 GB card and there's a good chance it will go higher. Someone else can answer that I'm sure (very large cards aren't important to me personally).
You know the FR2-LE doesn't have a digi-in, so using the USB-Pre's ADC with it is not a possibility.
Many here love the FR2-LE as a 1 box solution (and there are few owners besides me who regret the purchase), but IMO it has some notable deficiencies that the 661 does not share. It has a horrible headphone amp, worthless internal mics (which may not matter at all to many people) and I don't find it as convenient to operate as the 661. The headphone amp adds so much hiss that if you play back a quiet recording, you'll think the recording isn't worth keeping. The FR2-LE will record approximately 8 hours on an optional Tamiya battery as opposed to the 661 which goes about 4 hours on 2 AA MiMh's (but they can be switched out quickly during a set break). The FR2-LE will completely drain any batteries left inside within about 2 weeks, so using low discharge NiMHs wouldn't be much of an option. I can't tell you how much I hated that when I discovered it.