I've seriously thought about 64-bit Linux.
If it wasn't for World of Warcraft I would go full blown Linux. Gotta look at a dual boot system though thanks to WoW.
I'm considering test driving Win XP 64-bit RC1 as well.
I really want to learn this 64-bit CPU.
I got a sweet deal on the motherboard and CPU. The motherboard is a Gigabyte K8NS with the nForce3 250 chipset. I got it brand new off of eBay for $61 including shipping.
The CPU I got is an AMD Athlon64 2800+. I got it from ZipZoomFly for $127, including free FedSux 2-day air. It's in a retail box so it comes with a fan. The CPU will run at 1.8GHz, but it's FSB is 200MHz bumped to 800MHz due to it's system bus. The thing I will REALLY like is the memory controller is built into the CPU, so it runs at full CPU speed, not FSB. This has really helped with performance.
I'm going to eBay my MSI PT8 Neo motherboard and P4 1.8GHz once I switch everything around. I've also got a Dell P4 motherboard I'm going to eBay, plus my Samsung 4040R DTivo. I'm hoping to get somewhat close to break even point for the upgrade.