Well if you look at the figures in RG's database the Ultra II is some 0.0x something slower than an extreme III in a 20D, so I doubt you'd notice the difference in camera. IF you have a decent card reader, you might notice the difference in downloading to your PC, but many card readers have shocking performance.
Personally I'd go with the Ultra II, and either save some money or get a larger card. I use 1GB cards and sometimes really wish I had a 2GB...