Glad you got it working.
Just for note, a GTX 260 (whether the old 192 core or the new 216 core version) has 896MB VRAM, not 1GB. Also, GTA IV will not the memory you get from SLI. If you have 2x260s, you'll still see 896MB Ram.
As to why it would say 3xxMB for a GTX 260 I have no clue, but when you say 'Vista' I'm sure you've found the problem.