When I looked up the bike, the first thing that came up said your bike came with the "Soho S. Hardcase tires". I've ridden at least one flat resistant tire that had a TERRIBLE ride - I couldn't believe how awful this bike was to ride. So I would suggest different tires first - I use Panaracer TServ Messenger tires, they're still flat resistant but if anything they ride better than my old tires.
Secondly, as people have mentioned, running less pressure in the tire can help.
Third, running a bigger tire than 25c would
definitely help. I don't know how much clearance you have, but I've ridden a Specialized Globe bike that was 100% aluminum with bigger tires (35c) and it was a pretty darn smooth ride. 40c is was like floating. I've found that tires bigger than 28c seem to slow me down...but I'd rather be a tiny bit slower than be going through wrist pain. Another tire suggestion is the tires that come on the specialized I rode, if they'll fit your bike - I looked them up and they appear to be "Specialized Nimbus, 700x35C, 60TPI, Flak Jacket"
Fourth, if anything knobbies tend to make for a bumpier ride. A "slick" - a tire with little or no tread - would most likely be a better ride than knobbies, especially on pavement. I mean you're rotating a bunch of bumps around and around - it's going to be a rougher ride than a smooth rotating surface.
Fifth, I like the grips shaped like this better than the completely round grip - easier on my hands -
http://www.specialized.com/bc/SBCEqP...=41697&eid=731
I haven't used that particular grip, just wanted the pic for the shape.
Finally, I've ridden some cheaper front fork suspension bikes (and by cheaper they weren't actually that cheap - I rode a Cannondale Bad Boy with an integrated front shock, the bike was like $1,000) and was very unimpressed with the shock. My wrists still hurt and my hand were still buzzy. Your mountain bike is probably a better ride partially just because it has wider tires. Shocks are good if you're jumping curbs and logs - not sure they do much for reducing road vibration. You can replace the front fork as long as you get another one with the same measurements, but I would think a decent front shock would cost you so much you might as well just buy a new bike with the shock built in. I think the cheapest and most effective solution (though it will slow you biking speed down a little) is bigger tires. Also, all the high end straight bar aluminum bikes like yours come with either a steel or carbon fiber front fork which dissapate some of the road vibration. That's an option, though it's not terribly cheap.