OS uses about 1GB, plus or minus a few hundred, just idling at the desktop. Depends on what you have installed and what features you have enabled.
SC2 takes about 2GB or more on larger/busier maps.
Browsers take about 0.5-1.5GB, depending on addons, number of open windows, and private mode use.
Basically, if you're like me and disabled the page file (or dropped it to the minimum 16MB) you'll see a huge benefit: you'll actually be able to run a single program without out-of-memory errors.
If you have the page file extended on a spindle drive, you'll only really notice a performance bump on programs that use a couple GB at a time (mainly games or professional) or you're a multi-tasking fiend who is intimately familiar with the open window limit (about 50 in XP, 7 is higher).