IMO if you stick with AGP get the largest memory you can get because shunting resources to memory on AGP would be painful in comparison, for PCIe should you have large models then using the Hypermemory sharing system memory is much less painful and not as much of an issue for Autocad versus a game, but still I think get as large as you can for that class of card. The HD2600 is a solid choice, but even an HD2400 would be fine, however I think the limited vertex power of an X1250 or 1300/1400 model may leave you wanting, the X1600 would probably be OK, but likely still sluggish compared to even an HD2400 for your needs.
The other thing is everything lower than the X1600XT is limited to 1 dual-link DVI connection, and then for the X1600XT and above you need to see support per model. If you're at all considering adding another panel similar to the one you have, that should be considered. The X1650XT and X1950Pro both have 2 dual-link DVI connections, IMO just get the HD2K series and you're guaranteed the dual-link DVI and HDMI support you're looking for.
Also, IMO, get at least a 512MB AGP card, if you go PCIe you might be OK for 256MB, but yours is a rare case where memory size will matter more than getting an extra 50mhz out of the VRAM or core.