the iphone 3g wasn't monumentally better than the 1st one though? it had 3g and a few bits of trimming then it was the OS that made the differences which were translated throughout the range just like ios 4 for ipad.
apple products like the iphone/ipod touch and I'm sure the ipad will be the same are about evolution not revolution, especially when they have themselves in a very lucrative regular upgrade schedule. I had the original iphone when the 3g came out and wanted a 3g but didn't feel like I'd been done as the difference was gradual, same when the 3gs came out when I had the 3g and the 4 came out when I had a 3gs (yes I upgrade a lot but never on day 1).
There are a few facts that the tech rumour sites such as engadget etc all pretty much set on..
it won't have a retina display, way too expensive, uneccessary and too big a jump for app designers etc (and doesn't fit the whole evolution thing)
it will have a camera, cases and skins are already starting to pop up with cut outs for them but nothing on res (why you'd want to use the ipad as a camera absolutely defeats me, couldn't be less interested, phones yes.. I have one on me all the time so I get those "happening now" shots but the pad largely stays at home).
it won't have usb, they don't want it to be a macbook competitor so I'd expect expansion ports to stay exactly as they are, especially considering you can get the camera connection kit if you like.
it won't have hdmi as that conflicts with the airplay apple tv strategy.
so you're likely to get a ram increase (common complaint), cpu speed increase and maybe slight res bump and then maybe some weight saving (althoguh no size change, that's also confirmed by jobs I believe) and the camera.
I reckon you're very unlikely to get a storage bump, the ipad sits in the whole 16/32/64gb thing like the iphone/ipod touch and going to 128gb would be unlikely but thats just my opinion.
In short, I'd buy now. Its a great toy and unlike an iphone you're not tied into a contract (unless you're dumb enough to buy one on a contract) so if you like the new one, sell the one you've got and buy a new one! happy days!