Don't let opinions cloud the facts here, the OS isn't his problem. People shouldn't post advice in this section if they don't really know what they're talking about.
First check what you have in your laptop now, you don't want to buy 2x2GB and then realise you have a 2GB stick in there already!
Money has to stretch when your in Uni!!!
http://www.crucial.com/uk/store/listparts.aspx?model=Aspire%207720%20Gamer
you have 2 available slots, I presume there both filled with 1GB sticks. Maybe only one is used with a 2GB stick..
Try:
Using less programs while running CSS
Deleting unneeded programs/files
Defragment your HDD
Add more RAM
If they don't work, you wont have much choice but to upgrade in graphics.