You can try a lower video setting, but 1GHz is (was?) a great machine where most things began to work well. But I moved all the lab machines to 2.0 GHz P4 minimum and office machines to 1.0 GHz minimums.
You met the minimum so (excuse the words and I'm not slamming anyone/thing) so you get the minimum performance.
In closing, be sure the machine is pest free. Scan with Adaware, Spybot and Housecall.
I have a game, battlefield 1942, and I pass the min requirements. The game plays offline without any problem (sure the computer is a little stupid in the game but i cant play) but when i go online is when i run into problems. I get 60-100 FPS offline and barely 10 FPS when i play online.
CPU: AMD Athlon 1Ghz
Ram: 384MB SDRAM PC100
Vid Card: GeForce 5900
OS: WinXP Home
Internet: Cable at 3.5 MB/s