K7S5A

I was tring to upgrade my PC from Athlon XP 1800 (1.24 Ghz) to XP 2400 (1.8 Ghz) and after installation, it booted and everything worked well except that it was only showing 1.1 Ghz on both bios and comp properties. I removed it and reinstalled my old one back and first time I turned it on...CPU fan turning, monitor peach black, no beeps. I checked (disconnected/connected) all cables and power connectors and still same. I cleared the CMOS and tried again and this time the monitor came on with "ATTN ON SIGNAL CHECK INPUT SIGNAL CONNECTION OR FOLLOW POWER SAVE MODE HAS BEEN ENABLE" but still no beeps. I also tried XP 2000 (1.49 Ghz) and still no luck. I had my grounding bracelet the whole time I was doing these. Thermal gel was reapplied everytime I removed and replaced the CPU. What did I do wrong. Could this be a fried mobo or 3 bad CPUs or maybe bad PSU? It was running on 1 Gb (2 x 512 mb) DDR for almost 1 year now. I know someone can help me out there. I need all opinions/suggestions that I can get. Thanks......Ely