I have 512 MB on my Dell E1505, running both XP and Vista operating system in dual-boot.

Vista is really slow with only 512, but I expect that with 1 GB, it can be good for normal use.

I ordered yesterday a 1 GB stick from Crucial to boost my RAM up to 1.5 GB to run Windows Vista Ultimate.

The performance difference between the 533 and 667 MHz is little. Someone told me that you can have better memory timming with the 667 MHz ... But I don't know if this statement is true...

If you don't like Vista ( a thig that would surprise me ), you can uninstall it and reinstall XP, but you'll need your own activation key.