[Kalash]

There are SDHC drivers (I got from xda-dev) that are supposed to work with WM5 and 6.

If you're loading custom roms (namely the 6.X from ppckitchen) there is an option for SDHC or just regular SD card support.

The drivers should get you going with a bigger card.
I'm on 6.1 now - and have an 8gig with no problems.

I had it working before the upgrade, but I think it said it only had 4.6 gigs of space total - but maybe I was reading free space - and just thought it was wrong.

Just download, drag to your device, double click...
And you should be good to go. (Or look into "Cab via activesync if you're going to be doing a lot of .cab installation. It makes things too easy...)