Upgrades: $25 for 120G SSD (boot drive, goes in the empty second 5.25" bay), $27 for 16G RAM, $20ish for an E5670 6-core CPU, $35 for an LSI HBA in IT-mode (supports >2T drives), $10 for a set of SFF-8087 cables to go from the HBA to the existing hotswap backplane. 30-45 minutes to upgrade all the firmware for the DRAC, lifecycle controller, BIOS, etc.
Grand total of around $250 for a really nice server with full remote management, and for the cost of another E5670 and a Dell heatsink I can upgrade to dual-CPU (12 cores/24 threads).
They're so cheap that I've gotten two systems to upgrade as described, and this morning ordered a third (yet again $125) to have for spare parts.
The eBay vendor emailed me and offered to sell me a pallet of 24 for $80 each, but I don't have that kind of need or money lying around...
As for laptops, I tend to get refurb/off-lease Thinkpads from arrowdirect.com (coupon code ARROW gives 15% off), then max out the (cheap DDR3) RAM and throw a SSD in where the HD was. I've built up a T420s and an X230 like this for when I need a decent portable machine but don't want to take my expensive Macbook Pro somewhere. For the T420s I even got a $50 adapter board from a guy in China that let me put a FHD IPS screen in, instead of the 1440x900 TN LCD that it came with...