The dual core series are decent. On the plus side, they can run a 64 bit OS. On the negative side, they draw a lot of power, run super hot and are not at all well sited for overclocking.
I agree though, anything above a 3ghz P4 will suit you fine for MAME. Go cheap/free/Craigs list, you can usually find these machines for next to nothing.
Yes, you can get P4 machines practically for free. If cost is an issue, then by all means, go for it.
That said, I'm not sure where you get your information. According to Wikipedia, there have been 10 Pentium 4 3.0 GHz models and nine of them are 80-89 Watt parts. All of the low end Core2Duo E5xxx and E3xxx chips are 65 Watt parts, even the 3.0 GHz and 3.2 GHz models, so they don't draw a lot of power or run "super hot", especially when compared to the P4.
The Core2 Quads and Extremes are the ones that use a lot of power, but I'm not advocating them for use in a MAME machine.
There is also no problem with overclocking. I overclocked my E5200 2.5 GHz processor to 3.5 GHz using the stock Intel heatsink/fan combo on a dirt-cheap $55 ASUS motherboard without increasing the voltage and by changing only two BIOS settings. Most of the E5xxx and E3xxx processors will overclock similarly into the 3.5-3.6 GHz range, given the right motherboard and memory. Some people have gotten lucky and were able to overclock to the 4.1-4.2 GHz range, but that requires a better cooling and more voltage.
Any one of the low end Core2 based chips kicks the crap out of a P4. Plus they can run a 64-bit OS with 64-bit MAME for a 15-30% increase in performance in most games.
My personal recommendation for a MAME cabinet is a low-end Core2 based processor combined with a cheap LGA 775 motherboard from ASUS or Gigabye in the $50-70 range and 4GB of quality memory (2 x 2GB). Then couple it with a 64-bit OS, preferably XP x64 if you can find a copy on eBay.
For example a quick search of Newegg turns up...
$53 - GIGABYTE GA-G31M-ES2L LGA 775 Intel G31 Micro ATX
http://www.newegg.com/Product/Product.aspx?Item=N82E16813128357$50 - Intel Celeron E3400 Wolfdale 2.6GHz 1MB L2 Cache LGA 775 65W Dual-Core Desktop Processor BX80571E3400
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116348$80 - G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel Kit Desktop Memory Model F2-8500CL5D-4GBPK
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231166This combo for $183 would form the basis of a very powerful computer for MAME now, and probably for quite some time in the future. There's probably better motherboards for slightly more money, I just picked the cheapest highly rated LGA 775 motherboard that came up in my search.
With the extra headroom from the 1066 memory, you should have no problem overclocking the CPU into the 3.5 GHz range with the stock cooling. The memory is very important because most of the cheap LGA 775 boards do not have fully unlocked memory dividers so you need to be able to overclock the memory too, hence buying 1066 memory for a 800 motherboard/CPU.