Actually, you should keep 2GB ram so Windows remains happy and doesn't start swapping to disk. That is the path to darkness.
lol I hadn't turned my computer off for about a month, and this morning I was playing FTB Monster, while chatting on IRC and listening to music on iTunes. Turns out, when allocating 2.5GB of RAM, talking on IRC, listening to music on iTunes, all with the priority of MC set to -15/-20, will completely destroy everything. I was running with anywhere from 400-900MB of RAM available
Also, OP has 4GB of RAM, so 2GB of RAM is plenty. You shouldn't play heavily modded MC SSP if you don't have 4GB or more RAM.