I had similar specs to you although an x800 with a 22 inch widescreen. I found that my system just couldn't power that monitor properly and I built a new computer.

Having said that I had a lot of success player this game on the old computer with these settings.

Ran the game at 1440x900 (which is actually my desktop resolution is, 1650 is just too darn small for my old eyes).

Turn off all AA and AF. AA even at x2 will really kill your framerates at this high resolution with that older card.

In game turn off elevators (it's the planetary traffic), dust, asteroids and debris. All those things, especially elevators and dust really cause slow downs. Oh, turn off Bloom.

I then ran most settings at high and it kept frame rates above 25 most of the time but it would still slow way down in big battles but it was still very playable.

But when I tried Oblivion for the first time with this new monitor I had to upgrade as it just wouldn't run at all.

So for around $1200 I got a top of line system, Dual Core E6850 3.0 ghz and a top of the line Sapphire ATI HD3870 Toxic (factory overclocked). Needless to say my frames in Oblivion with everything turned up max rarely drops below 60 and Sins, well, I don't think I've seen any slow downs yet with everything cranked up to max.