If you do not have enough ram allocated you will experience "heartbeat lag" (You won't like it) I use 3 gigs and it's fine
~ @legendcaleb
The hell is "heartbeat" lag? Garbage cleanup and saving to disk are the only two processes I know of that can affect performance periodically, which is what I'm assuming "heartbeat" lag is; a periodic lag spike/performance drop. Neither of those issues are an issue with a lack of RAM, in fact, garbage cleanup can and will be made worse as you add RAM.
I run Vanilla at 1080p with a 32x Texture Pack. Default amount of RAM is more than plenty. Tekkit (110 mods, IIRC) requires no more than 2Gb of RAM, and that's very generous.
