No offnese bu t that's a poorly build design, nice video card though.
For starters, the motherboard. That Asus board is popular on the mainstream due to their name, and they make good board's for Intel CPU's. However Asus for AMD is crap crap crap, problems problems problems. I recomend a DFI Lanparty Ultra-D / SLI-DR in place of it.
For the PSU, you guessed right, a 420 watt will not really suffice, the minimum I would recomend is a good quality 480 watt PSU, from a company like thermaltake, antec, OCZ. Thermaltake gets my top recomendation for price however.
Last but not least for memory, 2 gig is not required and even had disadvantages. I can help you select some memory, but you should tell me if your overclocking.
And last but not least, the best to do is get a 3200+ venice core Athlon 64 from AMD, and overclock it. Your reaching FX-55 speeds easily without a problem, and with a DFI board and some decent ram almost anyone can do it.