Yes, in SLI mode the strobe multipliers are set to 8x on current SLI architecture. I beleive Crossfire works on dual 16x, not 100% though.
I have heard of dual AGP boards out there before, but they were few and far between and never seen in a desktop system.
SLI will only work with 2 identical SLI ready nVidia-based cards. Same model, same manufacturer. Crossfire allows for two non-identical cards to be utilised; an X800 XT, and the upcoming R520 series will/should work together.
AMD X2 4400+ Toledo 1MB/Asus A8N-SLI Deluxe/2GB Kingston HyperX/SLI Gigabyte 7800 GTX 256MB/Creative SB Audigy 2 ZS Plat w/ Creative Inspire 6.1 speakers /180GB Hitachi PATA/19\" AG Neovo F-419 LCD/OCZ PowerStream 600W PSU/WinXP Home SP2