Hmmm, the general opinion seems to be that NG runs slow no matter what.
I'm running on 2.4ghz dual core with 2gb of ram and a ham beast of a video card. I'm getting some lag, but not huge amounts so far. No need to worry about upgrading quite yet unless you really feel NG is quite that special :3
Or you generally just need a new computer.
If you feel your a little low on the moolah, This and this would slightly improve on what you've got currently for around $80, although it'd depend on how old the mother board and computer casing is. You'd also have to be willing to insert the new parts yourself (which in reality isn't very tasking).