Here's my advice. I've got a 2004 2500HD D/A, a 1999 F550 crew cab 7.3 (for sale as of Thursday), and a brand new 08 F450(purchased Thursday).
I pull a horse trailer with the first two trucks. My trailer weighs in at around 20-22K depending on horses and water, ect.......The trailer is quite a bit more than the 2500 should pull but it doesn't do to bad considering. The 550 pulls it great but no land speed records. I've got a buddy that pulls the same size trailer as mine with his new 450 and he said it pulls it like my 550 with a bit more power.
Now for the advice. You are a tad over on weight but not bad. The 3500 will do the job with room left to spare. If I had your truck and trialer I would stay with what I have and let them get a year or two under their belt before I purchased a 450. If I had your truck and my trialer I would probably not have bought a 450 the other day. Basically the only reason I did was for the 4X4.
Now let me tell you about the 450!!!!!!!!! They are SWEET!!!!!!!!!!!!!!!!! They turn great. Super roomy. Well thought out. When I turned it on the only reason I knew it was a diesel is because of the glow plug light that was on for about half a second. It is SUPER quiet. Great truck if it stays together. The only downside is the truck feels like it doesn't have much pick up or power. I think the new restrictions are hand cuffing these new diesels.
All in all you are pretty much set as you are. If you decide to get a 450 it will just cost you a bit of money. Mine set me back 53K.
Later,
Chad