(in descending order of "would make me take the rest of the day off from work to go to the store and get it")
More RAM (srsly, it hurts; only 128Mtotal/80MBuser is a bit low)
Faster CPU (it's serviceable as-is, but sometimes you can really feel it drag).
Yeah, those are really only the upgrades that I'd get in line for right away. They announce either of those upgrades next week and you can bet money I'd be offering my 4GB 1st-gen iPhone to whomever wants it (might still wind up upgrading, but it could take a week or so).
The other upgrades? 3g? meh. GPS? well, Geohashing could be fun, but honestly I don't have a need for it. More flash? I'm on 4GB currently (yeah, early-ish adopter that paid too much) and rarely fill it. Sure, sometimes I have to make decisions about what to put on it, but those times are not that frequent. Probably about once every other month or so. Though this might change with the new 3rd-party apps coming out (I haven't bothered to jailbreak my phone yet since I'm not quite tempted enough by any of the current offerings). I mean, Spore for the iPhone? Oh-yeah!