I make our organization (400+ employees) buy Dells. Optiplex line. Right now standards dictate Intel Pentium Dual Core, 3.2Ghz, 1GB Ram, and 160gb HD. Since we like them to last 3 years max, we max out the processor and memory. Memory is cheap to add later but might as well do it now. We're using the GX620s but looks like we are upgrading to the 745 line. We are just getting some of the Dual Core's, and they run pretty smooth, smoother than the HT's we were getting.

All in all, find out what exactly you are going to do (business apps, internet, email etc), find what you might WANT to do (ie games) and decide from there. These kind of machines aren't what you call, upgrade friendly.

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'02 X5 4.4 - 67k miles
'01 RX300 - Wifey's, 74k miles
'92 325i - My classic (Dinan S1 susp, clears, M3 kit, M3 exhaust, M3 Double-spokes, 216k miles)
'93 318is - Gave to niece