speaking from personal experience, i used to think a long time ago that the monitor didn't matter much and that simply having one was enough...
that changed when i upgraded my old 15" CRT monitor (it was way back then

) to a newer 17" CRT...
from that point on my view changed regarding how a better monitor affects your overall experience... it looked like a completely different and better pc... it even seemed to be faster
that happened to me again when i took the leap onto LCD monitors... i bought a 19" screen and was amazed at the difference it makes...
now i have 3 monitors, hooked to a triple head 2 go... so you might say i'm addicted to monitors now... ;D
so to answer your question... if you have an old CRT monitor, do upgrade it, you won't regret it... but if you already have a decent LCD screen, and you're getting jaggies, do as Jim said and tweak your resolution... usually the native res is the max setting on your desktop properties screen
hope i didn't blabber on too much ;)
Cheers
HarvesteR