I don't have any experience with dual cpu systems, but I would think that in terms of heatsinks and fans, all the same advice about single CPU systems would apply, especially if you don't have a power-hungry video card that would dissipate more watts than another processor, no? In fact, having two processors is probably easier to cool because they should both be in the main airflow.
What kind of board and socket are you looking at? I guess your options for modern processors in a dual socket sytem are limited to Xeon and Opteron. Are there any other dual socket configs? The Opteron would be preferable for its lower power consumption.
This was an enlightening read
for me. As you can see, dual-core isn't always a clear winner because a lot of apps are unable to use more than two CPU cores. I'm not sure what kind of research you're doing, but if it uses apps that take advantage of more than two cores, this would be the cheapest Opteron:
2 * Opteron 265 (S940, "Italy" core) ($email@example.com
) = $660
If not, this would be the cheapest single core Opteron:
Opteron 246 (S940, "Troy" core) ($166 firstname.lastname@example.org
CPU cooling - This is going to depend on what kind of motherboard you have and how far apart the sockets are. You may be limited as to how big of a cooler you get. Unless the sockets are really crammed together, an XP-90 is a fairly safe bet. If you have enough room, you can go for the Scythe Ninja with a quiet fan. Always spring for Arctic Silver thermal compound no matter what cooler you choose.
PSU - If you need to save money, you can get the P150 case that comes with the Antec Neo 430W. Otherwise, the S12-430 won't disappoint. You might not need 430W depending on what else you're powering.
Again, I think most of the advice on this site will apply to you, so read up