My take on things is that 775 is a great place to start. There are tons of different chips to bench and most if not all can be had for cheap. If you are going to be using DICE, 775 is also good because most chips scale really well with just DICE. Ln2 will take them higher, but one can still be pretty competitive with just DICE. Finding a good board is kind of the hard part, but a nice EP45-UD3P will get you through all of the core 2 chips and related Pentium models. P4 and celeron don't work well in most P45. One would need to mod the chip in order to get it to work. P35 and P965 along with X48 work well with P4 and celeron. Something like a P5B deluxe with mods will be a great board for P4 and celerons and the EP45 would be good for quads and c2d.
Now the thing with AMD. If you don't have Ln2, it's a waste of time IMO. Max clocks will be miles away from what can be achieved on Ln2 if you are only using DICE. For unlocked chips, AM3 boards are great. If you want to run chips with locked multi's, the AM3 boards don't take them very high and you're back to looking for a good DDR2 board. Other than that, I find the AMD stuff pretty boring for most of the 2d benches since AMD is so slow. But thats just personal preference. For AMD, the CHV is my favorite board. The ROG connect is miles better than anything giga can come up with. The board is also rock solid.