First off, do you get any 'beeps' when powering up your rig?
Your RAM might have gone bad on you. Try a clean start with bare minimum (one HDD, video card, 1 RAM stick, keyboard and mouse) and see if you can enter your BIOS.
If you still can't enter BIOS, try your RAM in a different computer (don't put another stick of RAM in yours yet because if your MOBO or PSU is bad, you might burn it too)
If you can enter BIOS, you may have to reinstall/repair your OS, since you replaced some hardware (i.e. motherboard) If your running XP, try repair option. Make sure your boot priority is set to CD before the HDD.
If everything boots up fine on the first try, reconnect your hardware one piece at a time until you find the broken part.
A slight burnt smell is not unusual when installing a new PSU. It might be due to some dirt burning on one of the filters.
My gaming machine:
P4 2.8 GHz
2 X 512 MHz PC3200 in dual DDR mode
Intel D865PERL MOBO
integrated 6.1 MAX audio
Logitech duo MX mouse and KB
ATI RADEON 9800 PRO