Yeah, I agree with Katya, go shopping with your girlfriend. Check out J Crew for lower priced (like $60-80) jeans - my fiance has a pair from there and they look so good on!
Also, think about expanding your horizons while staying true to your style. I love the Seth Cohen look, but it really works best on skinny guys with a certain kind of personality. My fiance would look like a humongous poser in those kinds of clothes and he'd be really uncomfortable. He's just more of a preppy, conservative dresser. It sounds like maybe you have a similar style.
However, I think I have been successful in dressing him a little better, and I know that he wants to have a little more fun with his clothes. I think I've helped on this by getting him to try slightly (key word, "slightly") more fitted button downs and tees - try a Ben Sherman shirt in a muted pattern, the fit is great. Also baseball tees I think look really really hot on guys - especially with nicely fitting jeans and a pair of brown leather flip flops. you might want to think about a pink or purple shirt even though you say no pink. Just try it - my fiance is becoming more receptive to this lately and he is definitely not metrosexual. Just make it a polo or other collared shirt in a manly shade of pink and try it on - I think on the right guy it can look very Polo/Lauren/upscale and not metrosexual at all.
Other ideas: flat front trousers - no pleats unless the pants are formal suit pants. A coroduroy blazer. For fall, wide wale cord pants (not too baggy though). The key is fit. No flannel or plaid button downs, especially not worn open over a tee shirt. too lumberjack/grungy/frat boy for my tastes. Shorts must be worn with caution - no jean shorts (i'm sure you know that already), and any shorts you do wear shouldn't be too long or baggy or too short either. Just some ideas...good luck and congrats on being brave enough to post here. can you tell, we love dressing our men.
I think this look is hot, hot, hot! And it's from the Gap, so it's easy to find and mimic.
I also dig this shirt - this outfit would be great for class or a casual dinner-date. (I like the look of two collars - very preppy cool.) All JCrew and Banana
I wish my BF could dress in the stuff that Maddie picked out, but he's a lot more preppy that that. His basics are Ben Sherman shirts, Lacoste polos, jeans (his staple pairs are A&F), and sneakers. He used to wear polos tucked into khakis until I started dating him.
in terms of some of the jeans that the ladies have mentioned- true religion, seven etc. eluxury has a selecton in their mens section to look at to get a feel for what u like, scoopnyc.com has some also
polo mens stuff can always be found for MUCH cheaper in stores like marshalls and tjmaxx. the filene's basment near me has a great mens section also,once u know what you are looking for there are some great deals- also i have bought tons of facconable(sp)for my brothers from nordstrom rack. (meanwhile you're girlfriend would know ALL of this too ;)