Computer fails to boot?

Hey all, so heres what happened. I overclocked my video card, an old GTS 250. While afterburner was open, my little bro messed with the settings and set my memory clock to the 1300 max and applied it. Panicked after the system froze and artifacted, he forced a shutdown and now it fails to display an image. I bought a GTS 450 from newegg, swapped it in and it still fails to boot or display anything, dosnt even POST. All LEDs and fans work, HDD spins up, the HDD activity light on my Antec 300 blinks rapidly as it would when loading windows. Ram is known good, their are 4 LEDs on my MSI mobo that display power phases or something, and they are lit, so not a mobo problem? PSU is a trusty antec 650 watt, dont think its PSU cause it only stopped working after the vid card problem. Help?