I recently got a 50D with UDMA support for CF cards, so I have been through this recently. I go the Sandisk Extreme IV, UDMA. There is an Extreme IV that doesn't support UDMA, so watch out for that.
I have a few observations about the process of getting the most from your flash cards.
1) you may need to replace your card readers to support the new UDMA protocols. Without a UDMA reader, you will be limited to something in the low 15MB per second range.
This one is cheap, small and very good.
2) I have a cheap USB expansion card in my PC. It was a bottleneck with my UDMA cards. I needed to plug into the USB port on the motherboard.
3) I have a RAID 1 system (redundancy) and its write speed is slower than the CF card reader, so I don't get the full benefit of the speed.
4) My old PC Card CF adapter, when coupled with my UDMA card, crashes my Vista laptop.