The Ti4200 performance is <A HREF="http://www6.tomshardware.com/graphic/02q4/021202/images/image026.gif" target="_new">far superior</A>.
Considering on <A HREF="http://www.newegg.com" target="_new">NewEgg</A> you pay another $2 for a Ti4200-64MB (MSI) over an ATI Radeon 9000PRO 128MB, it's obviously worth the performance.
Oh, and note, although the Ti4200 in that benchmark I linked to was a 128MB, that makes little difference in most games, the 64MB will perform similarily, and only lag behind when using 8xAS + 4xS FSAA at 1600x1200 or similar memory-gobbling situations.
-Col.Kiwi