In tests, under normal condititions an Athlon 64 is as quick as all but the very fastest (and obscenely expensive) P4s.
You will net get the best performance from an Athlon 64 until the 64 bit version of Windows & software is available, and obviously this will cost you more money on top of your system.
at the moment you do not need 1Gb of Ram, but it may come in handy at a later date. I have 768Mb and it's plenty enough for my needs. I even ran a full OS & office suite in Ram last night, with the hard drive unplugged.
So ultimately it's down to which system has all the bits & pieces you need, and don't think that buying the Athlon 64 will future proof your system, as AMD are changing the processor socket soon, so the same socket is used for the Athlon 64 and the Athlon FX.