I disagree with the rest, actually. I always used to be quite a 40k nut (I moderate the warpshadow forum, which is kindof the main Tyranid forum on the internet). I'm also a diamond-league starcraft 2 player, so I'm quite a fan of both! I'd love to think I'm not completely clueless in suggesting that the terrans would probably beat a marine chapter.
A single terran marine vs a space marine wouldn't necessarily be a stomping - they make good rivals, I think! First if we assume protoss psy blades are akin to eldar power weapons, giving protoss a conservative str:3 (which feels a little low for the mighty protoss zealots, and in fairnes psy blades are probably more akin to force weapons, but that would be crazy-strong), and let's also assume that zerglings are a lot like hormagaunts. We can conclude that a terran marine, able to withstand 6 hits from a zealot psy blade or 8 strikes from a small clawed alien, is less durable than an ultramarine (who can withstand 6 hits from an eldar power weapon, or 18 hits from a gaunt). BTW the ultramarine only actually withstands 3 hits from an eldar power weapon, and only 9 hits from a hormagaunt, but I'm going to include the 'miss chance' as durability, I think that's only fair.
In this comparison, the marine is considerably tougher, but the point is the two are *comparable* in combat. Space marines get seemingly stronger firepower (autogun vs boltgun), too, but terrans get stimpacks and combat shields, *hugely* increasing their combat skills! Overall I think it's safe to say that a space marine can beat a terran marine, but wouldn't have a chance against TWO terran marines!
Now a chapter of space marines consists of no more than 1,000 marines. Some chapters, of course, break these rules, but Ultramarines are famous, in fact they're pretty much DEFINED by, their Codex-compliance. Of all chapters, ultramarines are pretty much the most certified, sure and truly gauranteed to be only 1000 marines strong.
Terrans, on the other hand, can field more marines than that in a single battle! (6 players in a game together can produce 1200 marines). The terrans as a race can probably field a hundred to a thousand times more marines than the ultramarines can manage. It's not even close.
Going on from the basic marine comparison, there's:
marauders are more numerous, more durable, and more heavily armed than terminators.
Thors would absolutely destroy dreadnaughts.
Siege tanks are massively weaker than both predators and landraiders.
Valkyries, vikings, and dropships are weaker than thunderhawk gunships.
Battlecruisers are much weaker, but FAR more numerous than a battle barge (of which the ultramarines have 3).
Before anyone mentions them, ultramarines don't have any titans. No chapters do.
(Oh, and if you're going to be quoting exterminatus, then it's only fair to do an Arcturus Mengsk and call the zerg down on Ultramar, which is a much more powerful 'doomsday weapon' with far more dire repercussions for the space marines.)
Everyone here is already right about the numbers being on Terrans' side, but I think the respective power of spacemarine units is getting overestimated here, and the limited numbers of the spacemarines is definately getting underestimated.
Space marine chapters don't fight wars in warhammer 40,000. The space marine *legions* in the ancient past (and they *were* able to call upon titans for assistance) fought wars. 41st-millenium chapters are shock troops - they're like a squad of paratroopers or navy seals, they get in, take out a target, and get out again. They're neither designed for nor suited to fighting pitched battles in the way the terrans are.
Now if you were to compare an ancient space marine *legion* (the ones that were led by primarchs) to the terrans, I'd say it was a fair fight and an interesting question... (Numbers slightly in favour of terrans, power in favour of space marines).
A chapter, though, would be a cakewalk for the terrans, I think.