I was using rudy enb and maxed out at ~25 fps while looking at the saloon and chet's warehouse in Goodsprings.
There was one strange occurance where, while using rudy enb the fps seemed to stay around 40+. But I don't remember what changes or tweaks I had made before that.
Anyway, my system specs:
2.8 ghz AMD,
8 mb RAM
ATI Radeon 7770 HD
If anyone with a similar system has better performance I'd love to hear what settings you use.