The question will always be, what are you planning on using the computer for now and in 3 years? An upgrade is not always necessary. For the average user or even the guy that fires up photoshop once a week for simple edits, most computers today are overkill. If you plan on always upgrading to the latest OS or application software of course you'll see an advantage. The new software is designed to take advantage of it. If the old software really meets all your needs is an upgrade necessary? The 'need' for an upgrade is really up to you.
I say this because like you I recently bought a new iMac, 20" 2.6ghz core 2 2gb 320gb so similar setup.
I previously had my 2.4C Ghz Pentium 4 overclocked to 3ghz with 2gb ram, sata hard drives ect ect. (4 years old) I certainly didn't 'need' to upgrade to the iMac. I could have easily thrown in a faster processor, some more ram and faster hard drives for less than half the price of the iMac. I upgraded anyway because it was right for me.
To answer the other question if it will lag in a few years, most operating systems will. Great time to back up, reinstall the os and do maintenance and spring cleaning, your system will be as good as new again.