i have an asrock, and i can't wait to get a dfi. the ASRock is merely a cheap way for me to spread out spending money(even though it means spending more technically lol, but I'll find a use for it once i upgrade). I have an old 9600pro, once G71 comes out and HOPEFULLY lowers a few prices, I'll pounce on either a 7800gtx or probably an x1900 the way things look at the moment, and use that in the ASRock for a bit, but my plan is definately to get a dfi.
the big question is whether to sli/crossfire or not. Either way, you should be secure in your purchase of a DFI, especially as you want to overclock. that is what they're built for. ASRock are odd aswell, based o the fact that since bios v1.3 upwards, they've been removing OC'ing options. Plus, you have very lmited voltage adjustment, and I'm having issues with any overclocking at all now tat i'm using sata connections. As in, I could get 2x2.5Ghz, (possibly could have gotten higher, i was unaware of the HT multipier being too high at 5x at the time) and now if i go even up a mere 200Mhz, I get erors when loading windows, it insists a file is missing/corrupted. i can only presume this is because, like on that asus, sat ports 1 and 2 aren't locked, so they get overlcocked alongside the HTT, = BAD. and as there are no other connectors, i'm stuck with it for the moemnt. Also, the asrock has no vdimm adjustments, normal or high, high being 2.7, which is frankly low. also, there is only one sata2 connection. Also, the AM2 expansion card thing, it'll cost money, as would a new mobo... so unless it's super cheap, and somehow manages to still be competitive, you'd be better off ignoring that whole expansion thing as an option.
basically, keep the DFI, and go to DFI street for some awesome support/help/advice