I've been using the same Dell Inspiron for 3 years now. The only upgrade I did was install a 320Gb hard drive from an 80Gb that burned out on me. I installed Windows 7 and it still runs beautifully. I still use Photoshop CS3 on it to post process photos. I won't be able to upgrade it any longer but it does the job I need and should last another 2 years...hopefully. Not bad for $700.
I think the key is to buy the most ram and the fastest processor you can afford to stay with the curve 'cuz you'll never stay ahead of it.
At least a Core2 Duo and 4GB of Ram and you should be fine.