I don't recommend going higher than 3.1.3 for the 3G. The 4.x.x iOS doesn't run at optimal levels and has even been considered slow by a number of users.
If you're willing to take the tradeoffs of using 4.1, understand that it'll lock your baseband for 3G. Because you're on 3G, you might be able to reverse it, but I don't use 3G anymore so I can't guarantee that. You should do a search on this to be 100% sure.