One of the comst common questions people ask on Epic Games Forums is "will my computer be able to run UT3?" The official system requirements are listed below:
I think these are a little too optimistic for low-end computer owners. If you have onboard video with shared memory, it is doubtful that your machine will run UT3. And if you can run it with 512 Mbytes of RAM, you're in for some long wait times and frustratingly poor performance.
If you have the game already and want to get the best performance from your SLI machine, read my tweaking page. If you want to see how your system handles it before you buy, check out the free demo.