upgrade the CPU to dualcore
stick with the RAM you have
flash your BIOS to the most recent stuff
reinstall the graphics drivers
make sure your graphics board is defective free, because of the extremes in manufacturing computer boards, it is no surprise to find one board out of many that isn't doing it's job right.
do understand that a demo is intended to test (overall or particular) system/software/hardware performance, and is thus, not a legit way to say your graphics card is not working properly because you are not seeing fluid frame rates. programming for a demo, is considered a "beta" of what actually is yet to be released and thus the best of work is not finalized by the creators as their QA team still has a lot of press forward on with programmers alike. however, if the demo is old, try getting a real copy of the software.
-one demo is simply not enough, you need to do something like 3Dmark and other graphics applications.
keep your options open, don't settle for one graphics card if you're an enthusiast, consider your 1st, 2nd and 3rd choice of gaming hardware and see which one you can hold onto for as long as money permits. obviously you want the most juice out of a product if that's what you spend your time doing, so if you aren't satisfied with the Radeon 1900 series, maybe you should take a look at something else (or into the future).
hardware is not limited to capacity of what it is all the time, but how well it can cope with the software that is being supported by it/for it (summarized from Anandtech.com).