Is this a dedicated cruncher or your desktop that's also being used for crunching? The projects have minimum requirements for them as seen here: »www.worldcommunitygrid.org/help/ ··· nimumreq
. In all honesty it depends on what projects you run (some require upwards of 1 gig themselves) and what else this system might be used for.
Given you're only running 3 tasks at a time on this system, if it's a dedicated cruncher you could run any of the projects and be OK (it'd be pushing it though if you ran the 1GB projects). If this is shared with normal use, I personally would put more in it.
I've attached a screen shot for you to consider. I run a few servers at work that hose vmware ESXi and I have a Ubuntu VM running on each. I gave it 8 cores (same as the total number in the servers) but set it's weight to low and 1.5GB RAM. It's been assigned all 1.5GB but it's indicating the guest is using only 40% of the assigned memory.
Now, of those I mainly run the following projects: Help Conquer Cancer, Help Fight Childhood Cancer, and Human Proteom Folding. All of those projects utilize 250MB of RAM according to the minimum requirements. Simple math tells us that means I need at least 2GB of RAM for it and I could bump it up and see if it helps but if the stats are right and it's really only using 40% then it won't help.