Hmm, well not a lot of new games are coming out right now, I guess, but I'd say in about a year games will become fully Dual Core.
I don't know which one to recommend for you. A single core CPU will still last a pretty long time, but Dual Core will slowly become the standard. I guess it all depends on whether you want your PC futureproof or if you're ok with getting another PC in 1 1/2 years time.
But if you're aiming to build a PC for a specific game right now (Like Battlefield 2, really popular) then I'd go with the 4000+ which can be overclocked to FX-57 speeds relatively simply.
Then again, you could overclock the X2 3800+ to 2.5GHz and then that'd be like having TWO 4000+s, lol.