Does AMD even make a 3.0 Ghz Single-core?
I've seen that it's getting harder and harder to find a high-end single-core processor.
The question is: What are you going to use it for? If you have a specific reason why you would require even 2GB RAM, then that's fine, but the vast majority of the time I've seen someone wit more than 1GB the majority is left unused at all times.
I've been using a dual-core for a bit over a year now and I'm as happy as a clam; also satisfied with 2x512 DDR2 RAM running in dual channel. If you're going to pick up new hardware there are some things I'd take in to consideration:
You should get a SATAII (really SATA) harddrive. It makes a difference; a big one. Things will boot faster, there's less wait time, and by golly is my SATA harddrive quieter than my IDE one.
There's generally not much point in having more than 1BG of RAM, though getting even 2 gigs is cheap enough (just a few more bucks than getting 1, so there's not much reason not to.)
Power supplies: Expensive =/= good. Do some real searches about how much big of a power supply you will need. 500 watts is more than enough unless you're doing something very much outside of the norm. I believe that the first wave of cheap modular power supplies have recently hit the market. I remember Tiger Direct having some awesome deals on them (I believe they were Ultra brand.) Modular is nice because you only use the cables you need. New power supplies support SATA harddrives out of the box, so you shouldn't need to worry about adapters.
CPUs: AMD is generally cheaper and more power efficient but the high end AMD cannot compete with the high end Intel. Do not compare Ghz between the two, they use a different system (which irks me to no end.) If you're looking for value, I believe the AMD 3800+ dual core is the best bang for your buck (though there isn't necessarily THAT much bang.)
$$$: Always be on the lookout for bundles, rebates, and be sure to check the Sunday paper. Since AMD + ATI recently merged, you can find some good combo deals from them (say, a CPU and video card combo.) If you send in rebates, be SURE to make a copy of everything you send in. Tiger Direct is nice because they actually send you an email updating you (I just got a $50 rebate back.)
I hope you find what you're looking for. Just remember that you don't need to spend some obscene amount of money to get what you want.
Rrr, and to reiterate: I've had good experiences with dual core machines.