Thanks for the replies folks. The actual installed OS on this PC is XP. I just run backtrack from CD when needed, however, the performance in Windows XP is not my concern, it is what will work best for Backtrack.
So, judging by the responses here and my own testing, I am going with the DDR and I will throw the SD back in the my stockpile.
I doubt running slower memory will lower the CPU's clock speed. As far as I know those things are decoupled. Slower memory will just report to the CPU less often, but the CPU should be hitting it's onboard cache memory most of the time.
Off the top of my I'd expect the DDR ram to give the system an overall performance boost of around 10%. However if it ever tries to use more the 256megs it'll start using the harddisk which is about 1000 times slower than ram.
So with the 256 of ddr your best case is a little faster, whereas with 512megs of sdram your worst case is several orders of magnitude faster.
You just need to find out how much memory backtrack requires, if it's ever over 256megs go with the 512megs of sdram.