You can get some great manual Canon FD and Nikon AI lenses on ebay for quite cheap. I've bought a couple from this seller in Japan and they've been in excellent condition, exactly as described:
http://stores.ebay.com.au/japancamerama34/
You can get very cheap adapters from FD or AI to EF too.
If you want to spend a bit more I can recommend the Voightlander 20mm f3.5, very nice lens. But I guess this won't be too wide on an APS-C.
As for you main question, even my old 5DmkII was pretty noisy at ISO3200, even on a cold night but I guess with cooling you might get something usable. If the 400D is working for you and you don't want to spend money then keep using it (and the firmware hack is free), if you do want to spend money then IMO I wouldn't spend it on the 400D, not when you can get something like a brand new Sony A7r body for under $2k. Just my opinion