If you have the cash to drop right now, and don't mind spending it right now, then get the 360's. If you are looking to trim some money and want to still be able to play, just get some Supers or Comps and save yourself a phat chunk of change....see if you can get a layout diagram for the 360 so you make sure you leave enough room to upgrade in the future.
I dunno about NFL Blitz, but IIRC, someone posted here that Killer Instinct would play with ~2 Ghz pc, but take that for what it's worth, as I'm going off of memory there.
Still and all, I'm betting you can roll your own on a PC with a 3 Gig proc for less than that, after all, you don't need a case, sound card, modem/NIC and all the other fancy junk on a case. If someone else here can tell you if a 2 Ghz is fine for those two games you mention, then you can DEFINITELY build yourself an AMD machine that'll work and for WAY cheaper than that. The modular PSU isn't the way to go to save money, but if money isn't an issue right now, I'd say spend some money there. Reading quarterback, he just got darn close to the specs you think you need for WAY less money.
Help us help you further.
Do you have a budget for this machine that is flexible, or not. Meaning, does this need to be a money saver, is price not an object, or do you have an amount set aside to spend already that you want to stay within?
Next, you still didn't tell us what you mean by "tools". Do you have to buy them, and what do you need to buy?