Opteron 165/170 is a fantastic option, but that is only
if you can get it for significantly less than an X2 4400. The majority of remaining Athlon 64 X2's on the market will clock very much the same because they appear to be rolling off the same line. There's much debate about whether Opterons always clock better than their Athlon 64 X2 counterparts, but personally I believe there isn't much between them and there never really was.
If the X2 4400+ is just a few dollars more than an Opteron 165/170, get the X2 4400+ for the higher multiplier and stock speed.
I take it this must be the X2 4400+ you had in mind. Looking at the OPN code that newegg is selling, it's the "ADV" version. This has a low stock voltage of around 1.3V anyway. This chip isn't going to run any hotter than an Opteron. Chances are it'll clock very well too. Most of the ADV4400's clock well.
I'd also like to point out that when time comes to sell the CPU and move on, stock of all X2's would have dried up on the market and at that stage an X2 4400+ would have a considerably higher resale value than an Opteron 165 purely due to it's higher clockspeed.
Then again the Retail Opteron 165 is on the egg for $25 less and will surpass X2 4400+ speeds without a hitch so long as your motherboard isn't a duff clocker. Your call