How do you know the lag is not caused by a lack of video memory?
Also, a game that doesn't require much power such as Diablo 2 would easily get by with 512 MB of RAM. However, you have a laptop, which changes things. I suggest the following:
www.newegg.com
If you're a little bit more experienced (from the looks of it, I would just ignore this) you can look for specific things here:
www.pricewatch.com
XPbargains.com has some pretty good offers, but I prefer Newegg.