apvm I would definitely try the swap first. The stock Brisbane heatsink should be enough to run the Opteron at stock speeds, and the stock Opteron heatsink should be plenty to get a great overclock from the Brisbane.
boost, I can highly recommend the Thermalright Ultra 90 as an affordable heatsink with great performance that allows use of the first ram slot. You will have to install the ram first as part of the heatsink does sit over the slot, but high enough above that it won't touch.
For $16, this is a great deal (you'll need to add a fan though):
http://www.svc.com/ultra-90-k8.html
Now there are better performing heatsinks for sure, but none that I know of under $30-40. Also most of those heatsinks are probably the mammoth type that won't fit.
Now there is also the Thermalright-120 heatsink which is arguably the second best heatsink on the market. It is a massive heatsink, and probably renders the first slot unusable. However, Thermalright offers an adapter that lets you mount the heatsink sideways so that the ram slots are completely free. This is usually preferable as well because in most cases it lines the heatsink output with the rear exhaust fan.
Here is the adapter:
http://www.svc.com/s-clip.html
You can also use that with the Ultra-90. I'm going to verify for you guys that you can install ram in the first slot with teh Ultra-90, it certainly looks high enough.
Depends on the CPU boost, I have seen idle-load differentials between 3C (man I miss the PIII days) to 20C depending on processor and cooling. With our chips it seems to be 11-15C depending on conditions. However this is easy to test yourself, and I suggest that any testing you do be at full load. For overclockers idle or 50% load temps doesn't matter at all, it is wholly insignificant. If your system overheats at full load, then it will be unstable, and WILL crash at some point when you really need the power.
My suggestion is to measure temps with Orthos running Small FFTs. It will run both cores at 100% load and right now is truly the gold standard of stress tests for most overclockers.
SiSoft is good because it really gives you a good general idea of how much CPU power you are gaining through your overclock, it is satisfying at the end to do their comparisons and see your CPU match up to ones that cost 100s more. Other popular benchmarks include Everest, PC Wizard, and the venerable 3DMark (2001 is the best system benchmark, the rest are quite video card dependant).
edit: The Ultra90 definitely doesn't block you from installing ram in the first slot. I did it just fine, and I didn't even have to dismount the heatsink (kinda tricky though), there is about 1cm of clearance.