Actually you can SLI them. I'm currently doing it with success. My 9800GTX is the primary and my 9800GTX+ is the secondary. My drivers came up funky for a few restarts but for some reason XP went "Oh wait. Sorry, bud, I see the card isn't an 8800, my bad." My bridge was positioned wrong which probably had something to do with why my drivers came up weird after installing the second card.
Exactly the same experience here. Asus 9800GTX SLI'd with Asus 9800GTX+, working great! ... 'plus' card registered as 8800GT for the first couple of boots. Reloaded nForce drivers and checked SLI bridge to get 'found new hardware' notice ... nTune shows cards running at a clock frequency of 740Mhz (which is the 'plus' speed).