One lesson I've learnt over the years is that no matter how much RAM you put in a Windows machine.... a few months later you realise that it's about 50% of what you actually need to have in there.
My old Vaio C1MT got an upgrade when it was 6 months old, my PowerBook has a similar experience (with both of those it turned out that you could put in twice as much memory as the manufacturer said), the clunky old desktop machine just got maxed out (WinXP actually works a bit better now !), and... surprise, surprise my Vaio VGN-T27GP just got an extra 512MB thanks to the nice folks at the very aptly named Cheap Memory Australia.
Despite both of us being fairly careful to make sure we where ordering the right part for the machine, on the first attempt it didn't fit (my fault, I should have taken the laptop with me).... getting the problem rectified was (unlike some other returns I've had in the past) really easy (and I did take the laptop with me so we could test it) !
Great prices, great service, really flexible delivery (or pickup if you're on the Gold Coast) and a good choice of payment methods. Definitely one to bookmark (and they've got some interesting product).
Biggest improvement is just the general smoothness. On a typical day I have iTunes, IE, Outlook, SQL Server, IIS and various editors running away, as well as the various sundry helpful utilities and apps... and I've been noticing moments of thrashing while it sorts out the pagefile and physical RAM to display or refresh a window - since dumping in the extra it's behaved like a charm. Certainly the most cost effective way to turbo charge a machine when you notice it's slowing down a little.