I would expect a big improvement going from an Athlon 64 x2 to a Phenom II. HT normally isn't the weak link for speed. In any case, if you end up being unhappy with your current MB, you could always upgrade that later and keep the Phenom II.
Should be a pretty noticeable improvement, especially in apps that take advantage of quad core CPUs like media encoding or editing. Of course you would theoretically get a small performance penalty for the lower HT speed, but I don't think it would matter as too much directly as the big use of that HT link is to communicate with the video card and that cards communication is going to be limited slightly by that PCI-E 1.0 slot, but enough that the lower HT speed won't matter. Of course there is also the issue of not being able to crossfire with your current board as well as the issue of overclockability as the SB750 slightly increases the Phenoms overclocking abilities.
I really don't think that all those added together are enough to justify the expense of getting a new board so long as you don't plan on using crossfire and your system is running just fine. Seems ashame that you didn't go with an nVidia card since you have an SLi board. If it was me I would just use the Phenom II and wouldn't get a new board unless I could also afford another 4870 to take advantage of crossfire.