According to SiSoftware's Sandra, the overclocked mobile will be significanly faster than the A64 3000+ and 2800+ at stock speeds. An OC'd 2800+ should of course be faster than the 3000+ provided you get atleast 100mhz out of the oc (I've no experience OCing an A64, but that sounds easily doable).
Personally, if money was not a big issue and I needed a solution now, I'd get the A64 2800+ and oc it. This time next year I'd then be able to upgrade to the A64 3800+. That would be a significant upgrade and I wouldn't feel so bad about missing out on Socket 739.
However, I don't think that the Socket A procs aren't dead. I've seen plans that indicate AMD's going to continue on up through to 3000+ with their mobile line. If they continue to overclock as well as they do now, that too could be a nice upgrade as well.
Either way, you'll want PC3200 DDR400 RAM to match your proc's 200mhz clock speed. However, if you plan to overclock, you might want to move up to PC3500 DDR433 because it defaults to a 216mhz clock speed and should then allow you to get an FSB of 220 or higher if you're proc can do it. That's not necessary to overclock, you can always up the multiplier, but a higher FSB will produced even greater performance gains.
I'm actually a computer noob though. I just put together my first system a few weeks ago, an overclocked mobile barton, and while it's going well, I've still a lot o learn. All that's to say, don't take my word as final.