If you're buying an X2, look at the Dual Core opterons. Same steppings as the 4400+ and 4800+ with the 2x1mb cache, but are clocked lower and are a lot cheaper ($200 cheaper for the 1.8Ghz 165). They also overclock very well on stock voltage to get up to the 4400+ or 4800+ speeds with the extra cache unlike the 3800+, 4200+, or 4600+. I actually swapped my 3800+ for a 165 and it works great (the 3800+ worked great too, but I have the extra cache now ).
The opterons seem to clock higher on stock voltage. I put mine in and went right to 2.6Ghz with 1.4V. To me it was worth it because I picked up the extra cache and an extra 100mhz (I was running 2.5Ghz on my 3800+) for the same price if not a little cheaper than the X2. I basically broke even in selling my X2.