I say the x2 3800+. If overclocked to 2.4 ghz, it will perform the same as a 4800+ and it is not uncommon for people to get 2.7 on good air or water-cooling.
I found a review that overclocked the 3800+ and compared benchmarks to the 4800+. All in all they performed pretty much the same, although the 4800+ beat it slightly in a lot of areas but also the oced 3800+ beat it slightly in quite a few others. I will try to find the link and post it.
i mean, you can get a single core 1MB L2 and get a duel core 1MB L2. so in reality, if a game only ultizes on one core, it only gets 512MB of L2 as opposed to a single core would get 1MB