Along the lines of what has already been said, the FX-53 is an $810 2.4 GHz SledgeHammer on Socket 940. The 3800+ is a $715 2.4 GHz Newcastle on Socket 939.
Personally, if I were building a high-end system, I would be getting the cheapest Socket 939 retail CPU available. That's currently the 3500+ -- a $499 2.2 GHz Newcastle. Are you going to notice the difference in anything you do between a 3500+ and a 3800+? I don't think it's worth another $216.
The ASUS A8V is the Socket 939 board that's available. It's for 4 non-registered DDR slots, supports dual-channel DDR400, GB LAN, SATA RAID, etc.
You can get it
here. That's the Deluxe model for $169 that comes with 4 SATA cables and a Wireless-G PCI card.
I'm a big fan of the Enermax power supplies. The Enermax Noisetaker AX PSU's (AX are the ones with active PFC) are by far the best power supplies I've seen. You'd be much better off with the 370W Enermax than a 500W generic PSU. The 370W one is $83 and the 470W one is $105.
Anyway, as far as the price, that totally depends, but the machine I would build is:
Athlon 64 3500+ Newcastle Retail: $499
ASUS A8V K8T800 Pro socket 939 board: $169 (at ajump.com)
VisionTek Radeon X800 256MB: $423.95 + $8.30 shipping (at axiontech.com)
2X 160GB Hitachi SATA Deskstar 7K250, 8MB cache, 3-year warranty: $103.50 x 2
2X 512MB OCZ Enhanced Latency DDR400: $128 x 2
NEC ND-2510A 8X DVD-R / DVD+R dual layer: $89.99
AMS gMono polished black ATX case: $42 + $15 shipping
Enermax Noisetaker AX series 370W PSU with active PFC: $83 + $6 shipping
Zalman 80mm case fan with quiet mode adapter: $4.99 + $4 shipping
That's $1,808.23 total with shipping (all prices are from newegg unless otherwise noted)
A 470W PSU will cost $22 more, and a Thermaltake case will cost about $60-80 more. A good Conexant V.92 modem is $12.50 delivered. You could easily get everything except your monitor and speakers for just under $2000.