That said, for personal use at home, what would be recommended to build for better MAME compatibility, if not X2? I'd assume with the same 7900/7950 but a better overall PC I'd be able to support the same X2/NESiCA titles plus more MAME titles.
I have a PC in my Kraylix cab that has been running TTX games and many emulators for a couple of years now...
Most emulators don't support multi-core processing and most that do don't benefit from it much. so for emulation in general the thing you want most is a high clock speed. So if money is no object, by all means blow your cash on a high end i7 or i5, but if you're trying to do this on a budget going with a 3.4GHz or higher i3 or G-series works well. Some of these cheap itel CPUs can be overclocked to over 4.0GHz and outperform (in terms of emulation) chips 3x their price.
As for the GPU, more important than the speed of the GPU is it's compatibility. TTX games mostly use DX9 and in my experience fare better on nVidia GPUs. for Demul and GC/Wii you want a DX11 compatible GPU so that basically locks you into Win 7 or higher and later model GPUs.
MAME doesn't utilize the GPU except for post processing (such as HLSL crt emulation, and scaling) so any GPU is fine really as long as it's not a motherboard based "on-board" processor (most on-board GPUs leach performance from the CPU so it will hurt MAME performance without a dedicated graphics card).
If you want to run the bleeding edge emulators (like those for PS3, WiiU, and Xbox 360) you need a DX12 compatible GPU which will lock you into Win10.
I don't run Win10 myself, but I did have LOTS of compatibility problems with TTX games on Win8 and ended up downgrading to Win7 to get them playable.
Unless you're getting into PS3 emulation (which is still pretty rough) PCSX2, Dolphin and Demul (NAOMI2) are going to be your most demanding emulators so you should target your system for those.
as for the rest of the system:
8GB of RAM is more than adequate,
put the rest of your money into Hard drives... a complete PS2 set alone is over 2TB, Space gets filled fast especially for newer systems. Also make sure your mobo has some USB3 ports and Gigabit Ethernet because transferring lots of files takes forever even with these newer faster protocols.