No No No..
The P4 is an older chip architecture, it's not all about clock speed. To be safe, you're going to want to get yourself into LGA755 (Core 2/Core 4) territory. You can start off slightly cheaper with a "dual core Pentium" 755 series chip if you want to save about $100.
It really depends on what you want to run. If you want to be at the cutting edge of MAME and be running 3D graphic-heavy games like NFLBlitz, Crusin USA/World, and Gauntlet Legends, you're going to need an E8400 or E8500 (core 2) or a Q6600 (core 4), overclocked, running a 64 bit version of Windows XP or Windows 7. You'll have hit a ceiling going this route, however. If you want to be sure you can upgrade in the future without replacing the motherboard, you'll have to get yourself into the i3/i5/i7 architectures and that chipset.
This is the definitive topic on benchmarking MAME vs CPUs:
http://forum.arcadecontrols.com/index.php?topic=72776.0As far as the "best emulators" list, I think there have been many threads here but I don't believe there is a complete list of "the best" for each system. Best bet as far as Maximus is concerned is to use their list of supported emulators. Most of the emulators for older consoles are pretty complete, so I don't know how important the choice is. For GameCube/WII dolphin is your only choice. For newer handhelds like the DS, there's a few out there and they have different compatibility levels. I Believe there's quite a variety of PSX and N64 emulators too that vary pretty widely, so you'll have to do some research there.
Also, take a look at RomLister (see the BYOAC Wiki pages on it) for making your game lists in Maximus, at least for MAME. Also, make sure that you filter your roms. You really don't need 10 versions of each rom in your game list. The software forum here is excellent for topics on doing just that.
Good luck with your system!