Previously, when I went from 512MB to 1GB, the only improvement I found was in UT2004 - it had a few irritating jerks that I had put down to the Graphics and it still happened with a new card. the 1GB improvement was the cure to that.
When I went from 1GB to 2GB, again, I felt no true benefits... Not really.
When I went from 2GB to 4GB there was absolutely no benefits at all in the general use of the system for every day tasks, and in truth I did need it for VMs but not for much else.
Now, things are different these days... the difference between 2GB and 4GB is fairly substantial for those who use their PCs... If all you are doing is browsing and sending the odd Email to aunty flo then no, there wont be any Vital use at all in going for more than 512MB on XP or 1GB on Vista, but if you fancy running more then 1GB is a vital minimum for XP and 2GB for Vista IMO
Is there anything in going from 3GB to 4GB however?
In fact, some might say that the mixed modules will slow the system down and that a matching set will be a better option, but in truth as long as the RAM is near to *** same, it should not make any difference to the performance at all, and that the only real benefits will be that extra 1GB... Do you think that you are doing anything that will justify the extra 1GB?
IMO these days, there is absolutely no excuses for running less ram than you need to.
I own a fair few systems, 7 of which are permanently setup on my LAN and even my 3 kids ( 9, 10 and 10 ) are all running 4GB now and my daughter only really plays Little Mermaid and MSPaint more than anything, so while thats total over kill I know, the price of RAM justifies it. ( Plus if I need some RAM desperately, I know I can rob from her without her knowing anything about it )
Anyway, I am just waffling on here... End of the day 3GB is plenty and upping to 4GB while seeming a luxury now, you will no doubt be needing in the very near future, so just get it and have done with it.