I think you'll have to buy a new Motherboard and RAM to run that processor. You'll want at least 512Mb of PC2700 to run a 2400+ (2GHz) and then you may also want a new HDD. Since your not worried at all about graphics, keep the GF2 MX.
BTW, since your not playing games what are you doing that will need more power? If your just surfing the net or writing documents then theres no need for an upgrade, apart from perhaps a little more RAM.
Thats $352 (plus a little shipping) which leaves you room for a new case if needed.
I know you said you didn't need new video, but going from a GeForce2 MX to a Geforce4 MX for $10 is probobly worth it.
Hope this helps a little.