Either of them should overclock VERY well. They only come in E0 and G1 stepping, I have a Celeron D 320 with an E0 stepping and its running at 3.8 on default voltage. I'd imagine that the G1 overclocks just as well if not better.
The only problem is that you're going to need to be able to run 233+FSB for 3.8+ghz. So if you're that interested in hardcore overclocking it, look for ram that can handle the FSB 1:1 like PC4000.
I think its a win-win situation with the Celeron D 315. No matter which stepping you get, its going to be a good overclocking one. Post results when you get everything up and running, I'm very interested in this, especially if you get a G1.
Oh shit, I just realized that the 315 runs at 2.26ghz. I was getting it confused with the 310 that runs at 2.13ghz.
Intel made those in C0, DO, ane E0 cores. Looks like you got lucky and got one of the latest ones built using the E0 revision. I still think that you'll have little trouble hitting 3.6+ghz.