Me thinks you misunderstand,
by today's standers a 1.5 gig processor is darn pretty slow. In fact it's so low-end it would be hard to find in a new system. Unless of course you live in a 3rd world country.
I'm not suggesting you could get away with a 333 and run all the games you want, but you don't need a 3 gig machine to run these programs.
That might have been what you meant to say, but that isn't what you said. Go up and look at your post, you pretty blatently implied that a 333mhz is all you need by the way you worded things.

Regardless, as of .106u3 the bar has officially been raised. People seem to think that these new dx8 core changes are supposed to increase speed and functionality. Quite the opposite really. If anything the new changes will make things slower on lower-end systems. What we get is increased functionality (and that is a HUGE understatement).
While you might not need a 3 gig processor to run some of these games, a 1.5 isn't going to cut it anymore. (Even though I eluded to that and I'm sorry.) The range is gonna jump to around 2-2.5 gigs, which is around the specs of a typical middle-of-the-road pc you would buy new today.
And actually, as of .106u3 you DO need a fairly powerful video card. Not top of the line mind you, but fully dx8 compliant. (Meaning radeon or better). You can still run mame without it, but you won't have the benefit of hardware accel, meaning things will be slower than they could be.
I'm by no means saying buy top of the line either, or even new for that matter, but you can't get away with scraps if you want to play Killer Instinct.
Anyway, this guy says he is new to building computers (meaning he doesn't have spare parts lying around). Because of this it would be impossible to get a machine for less than 350 without either buying the system, or some of the parts used. Ram alone is 100-175 bucks if you want a gig (which you don't need unless you are preparing for vista or something). Wireless nics are fairly expensive and it's hard to get used cards because the tech is farily new. We haven't even gotten to the actual pc yet.
It's not a lost cause, but a more realistic figure (assuming you want to buy all new parts) is 450-500. Now you can get a pre-built pc for around 350 or less, but it's probably going to have a video card you don't want so there's another 50 bucks and all of these pre-built pcs need more ram then they come with so that's another 50-100 bucks. And I'm not talking about a brand-spanking new 3 gig+ pc, I'm talking about whatever is on sale (probably around 2.2-2.5 gigs).