When using ESRI desktop product from release 8.X onwards I have found that it is best to buy the best machine budget can buy as they are very resource intensive.
Using ArcSDE will limit some the spatial processing that must be carried out locally on your machine but does mean a good intranet is required as the data is streamed over the network.
You should be thinking of 512MB DDR RAM, 2.0GHz chip for fast easy use. If you intend to use much raster image processing this specification will need to be increased.
I personally develop in arcobjects using 512 RAM and 1.4 GHz chip and find it a little slow at times. So for real complex analysis work the best you can afford is a great idea. Unless you like progress bars.