depends on what you want from it, my old 3700+ was faster with games than my old X2 4200+ and it has about the same proformance in games as my X2 4400+ i have now.
If you dont do very serious gaming very much, or your not a prefection freak where every little choppy frame buggs you, then the X2 3800+ is the best choice. Also if you just like running multiple apps in windows.
I'm not sure about the X2 3800 but i know my last two X2s have'nt been very good overclockers, but my 3700+ made it to 2.8ghz stable, so far ive only been able to oc the X2 4400 to 2.4ghz.
DFI infinity RS 482
3Gig Corsair PC 3200
AMD X2 4400+ 2.4ghz
250Gig Samsung SATA 3.0 7200rpm
250Gig Hatachi SATA 3.0 7200 rpm
60Gig Maxtor ATA 5400 rpm
2x160Gig WD SATA 7200 rpm RAID-0
ATI X850XT 256mb
Antec TruePower 430