Hey all,
I just bought this processor and motherboard and want to make sure (yeah, AFTER I bought them...) they are compatible.  I was sure they were, but then something confused me, I'll explain in a bit.  
HERE is the MOTHERBOARD:
ASUS "A7N8X-X" nForce2 400 Chipset Motherboard for AMD Socket A CPU -RETAIL 
(newegg.com)
Supported CPU: Socket A AMD Athlon/Athlon XP/Duron Processors 
Chipset: NVIDIA nForce2 400 + MCP 
FSB: 400/333/266/200MHz 
RAM: 3x DIMM for DDR400/333/266/200(Non-ECC,unbuffered) Max 3GB 
IDE: 2x UltraDMA 133 up to 4 Devices 
Slots: 1x AGP 8X, 5x PCI 
Ports: 2xPS2, 1xCOM, 1xLPT,6xUSB2.0(Rear 4), 1xLAN, Audio Ports 
Onboard Audio: Realtek ALC650 6CH w/ built in HP amplifier 
PROCESSOR:
AMD Sempron 2400+ 1.667GHz, Socket A Processor - Retail
- Specifications -
Manufacturer: AMD
Model: SDA2400BOX
Core: Thoroughbred
Operating Frequency:1.667GHz
FSB: 333MHz
Cache: 256K (exclusive) L2 cache; 128KB L1 cache
Voltage: 1.65
Process:0.13Micron
Socket: Socket A
Multimedia Instruction: MMX, SSE, 3DNOW!, 3DNOW!+
Packaging: Retail (Box with Heatsink and fan)  
 
Now I looked at both and went ok Socket A and Socket A, should work fine.  What confused me is if you go on newegg and pull up motherboards and look at the ASUS boards for AMD processors, you will notice that all the boards that say they support the Sempron processors state they are socket 754.  Yet the Sempron processor I bought clearly states it is socket A.  Am I missing something here?  Is newegg out to get me?  Am I losing my mind?  Somebody get me another beer......