If it was me, Id pick the dual core to be more future proof. The 4200 would be a sweet spot choice right now for X2s, and they overclock nicely. The difference in the 4200 and 4400 is the extra cache, and i dont think its going to provide any significant difference. But hey, I love my 4400.
As far as problems go, the incompatibilities with older games can be fixed by setting the affinity through the task manager (CTRL+ALT+DEL). You will basically run the processor as a single core instead by turning off one of the cores for games that do not support dual core like HL2 and GTA:SA. You can search all over the web for how to set affinity for games not supporting dual core. So in a sense you get the best of both worlds going dual core, and they're cheaper these days, and new games like Oblivion will utilize the dual cores more. Make sure to check out AMD's website to get updated dual core drivers and the microsoft's dual core hotfix at their site. I dont have links to them anymore, but they will help a lot if you do decide to go dual. And when you go dual, you never go back.
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AMD X2 4400+ Toledo
XFX and LeadTek GeForce 7800 GTX 450/1250 in SLi
DFI Lanparty nF4 SLi-DR (704-2bta BIOS)
2X1GB OCZ Platinum DDR Ram (2-3-2-5)
Hitachi Deskstar SATAII HD 250 GB
OCZ PowerStream 600W SLi-Ready Version
NZXT Guardian Case
Creative Sound Blaster Audigy2 ZS
Creative Inspire P7800 7.1 Surround System
Samsung 204b Black 20.1" LCD (1600x1200)