I got one of these at the beginning of December...paid about $400 for mine, after tax. I got the free memory upgrade, and paid $100 for the processor upgrade to a 2.8G HT P4.
This thing is amazing...much nicer than the 400SC, in my opinion. I'm buying this as a very long-term keeper, so DDR memory and SATA hard drive are both big pluses, and make my upgrade path a year down the line so much simpler. Also, I expect PCIe video to be very, very affordable in 6 months - 1 year, so I plan to pick one up and do the slot mod at that point. For now, this system's running 2 21 inch monitors and one 17 incher just fine with one crappy video card + one 128mb PCI Radeon. A 128mb PCIe card later will give me 4 21 inch CRT's on two dual head cards this summer, and a possibly broken desk (there's an aquarium on it, too).
I'm running XP Pro, no driver headaches or install issues yet. Performance is rock solid stable (with half a G of ram) and 3D work in lightwave is great.
I would recommend a double memory deal over a processor upgrade deal for this one, since ECC DDR2 is still very steep. If I was going to upgrade to 1G of RAM, I would probably do it by getting another double memory deal on one of these for $234, with the hard drive upgrade to 80G, so say about $250 with taxes. Then I'd try to sell the box, celery processory, motherboard, PS, etc for $200 (is this reasonable?) and spend about $50 net for a half G of ram and another 80 G drive. Or maybe outfit the leftovers with older hard drives, DDR ram, etc, and sell it for more...I don't know.
-D.