I have an old NVIDIA GeForce GT 430 1 gb ram and 123D runs fine with it.
What ever you get make sure it has the right interface for your motherboard.
Newegg.com - NVIDIA GeForce GT 430
Search Results for GeForce GT 430 at TigerDirect.com
Here is the system requirements they suggest you have, be sure you also meet these. Getting a new video card to run something and then find you processor or memory are lacking is a real disappointment. Been there done that.
Windows 7 (32-bit or 64-bit), Windows Vista® (32-bit or 64-bit), Windows® XP Professional or Home Edition (SP3), or Windows XP Professional x64 Edition (SP2), Windows Server 2008
Intel® Pentium® 4 or AMD Athlon® 64 or later with 2 GHz or faster processor; or compatible
1+ GB RAM (512 MB minimum)
1.5+ GB free disk space (for installation)
Direct3D® 9 or 10 graphics support with 64+ MB