Ok, so I can't say whether latency could be an issue, but I suspect not - I think, no matter what solution you used, latency would be so low as to be irrelevant.
Positional cues seem to be an issue for you - that means you want the game to output a 5.1 or 7.1 feed, so that the positional information that the game is capable of providing is there in the output you take from it.
Forget the "enhancements" which you say you won't use - that's not really the point, the whole point is that if you want positonal cues in 2 channels, you need to start with a source that actually has those positional cues in it, and that means 5.1 or 7.1
You can then feed that multi-channel feed straight to a pair of multi-channel headphones with actual discrete drivers in them for each channel, so there is more than one speaker feeding each ear, however the problem there is that from what I've read they tend to be quite poor overall, since there are big constraints to trying to fit so many drivers in each headphone (each driver needs to be smaller which further restricts it's performance). Also, I think the whole headphone enclosure around the ear and the small distance from the driver to your ear would reduce the surround effect by a lot compared to speakers. Most stuff I've read says that, counter intuitively, 2 speaker headphones can actually produce a better surround effect than "true" 5.1 or 7.1 cans.
If you're going to use 2 channel headphones, with only a single driver for each ear, you need to take the 5.1 or 7.1 output from the game, and turn it into 2 channels, but you need to do it in such a way that you still get the positional cues - in other words, you need a set of effects that give your brain cues (like delay, echo etc) that make it think sounds are coming from particular spatial locations. So the sound card takes say a sound coming from behind (which it "knows" should seem to come from behind because of which channel it came in on), adds reverb/echo to it, and does other clever stuff to trick your brain into spatially locating that sound behind you. That means something like Dolby Headphone or CMSS3D which I'm sure the sound cards you're looking at will do. This is what you really need the sound card for in your case.
Just from looking at it quite quickly, the ZxR looks really good - I think you'd be pleased with it. I do notice that all you get with the ZxR over the next model down is a"DBPro" daughterboard which gives you ADCs (for recording) and also gives you optical out (I can't actually understand why that wouldn't be in the basic card, but whatever) which you might not use, but still, I am guessing you'll want all the options available so you could try an external DAC later if you like.
I also notice that the opamps on the ZxR are in sockets so you can change them - that affects the sonic signature, just like a separate headphone amp would. The opamps are preamps - they are the analog part of the DAC portion of the card, and they are what outputs the line-level feed.
HOWEVER (and it's a big one) I would not touch a creative card with a barge-pole. Please do feel free to ignore me, and if you do I genuinely hope you find things have changed, but I know their driver issues have been absolutely appalling, and I don't have any reason to believe it'll be any different with new cards. Their support is awful, and it could well make a potentially good (and rather expensive) card a lemon. I expect there is quite a lot of user-community support out there, and there may even be open source driver fixes, I don't know much about it because I avoided their cards because of the issues, but it's definitely a big area to look into before you drop top dollar on one of their cards and find it's not what it should be because of poor drivers.
With that in mind, and considering you really want good quality headphone output, the Xonar Essence STX seems like a good option -
http://www.stereophile.com/content/asus-xonar-essence-ststx-soundcards-follow-september-2010 suggests the headphone output is up there with expensive headphone amps.
I'd recommend sticking with just a good sound card (that comes with a headphone amp) or a good card + a modestly priced headphone amp if the card you choose doesn't come with one. That will then leave you with more money to put towards a headphone ugprade - unless, like me, you also have good speakers, I'd make that priority #1, and if you plan to do all your gaming and listening on headphones, I'd so some research into good, non-gamer headphones. Gamer headsets tend to be pretty junky just because of the demographic they are marketed at, you seem to have one of the better ones (as do I) but you'll find a genuine hifi pair of headphones at say three times the price will be a whole different kettle of fish.
