How about putting in the correct Forum... Try Hardware!!

Assume that all the brands, OS, and applications is the same...
Same motherboard also?

Your question seems to be more:

Will increasing the on a given system memory to 512MB make the system Faster??..

NO... well not really.. just depends on the application you are running.. but then some will differ.. Som Apps will Looooove the extra mem.. others couldn't careless..

Now changing HDD's can make a big difference on HDD intensive operations
memory speed "Can" BUT only is the CPU and MoBo supports the FSB required..
Oh did I mention that the Motherboard can make a difference.. not just the FSB but also the manufacture, the chipset used etc..
Give tomshardware.com a go as cyclops07 recommended.. it is a handy site..

Cheers