PDA alternative to TI Voyage 200?

Not sure about the TiV200, but the Ti83/86 is easily obtainable for PocketPC and Palm. Personally found it to work best on my PocketPC.

Any touchscreen based PocketPC would give you the most options if you're willing to pay and finding 300+mhz Xscale/Arm/etc models isn't hard there's a ton of 3 year old ipaq models out there.

However if it's the only thing you're doing with it you could use a cheap Palm Zire.

Just make sure you find the software you prefer first. Finding cheap capable PDAs is the easy part.

Heck I'd almost sell you my UTStarcomm as a 321mhz PocketPC if it weren't for the white screens of death that plague it.