Ok, so long story short, I picked the wrong day to mess with my PC's innards. After I was done, I started having this issue:
My first instinct was that the video card have bit the dust completely (it was already on the way out anyway, glitching intermittently with graphics intensive games like Borderlands 2). But the problem persists even after I swapped the cards for several other known good spare (albeit older) cards I have.
Any idea what else could be wrong? I've tried reseating the memory bars as well as doing preliminary tests on the system and the problem persists. The problem just suddenly started up. I didn't really do much to the innards, just installed two removable hard disk drive trays and the only time I (barely) touched the motherboard was to change around the SATA cable connections.
Motherboard is Asus M4N98TD EVO, video cards are two Asus ENGTS450s. Spare cards used to test the board are an Asus ENGT210 and a Leadtek GT520. RAM are 4x2GB G-Skill 1600MHz. CPU is an AMD PhenomII x4 925.
I'm really confused now and any help is appriciated- the problem appears to be graphics card related, and yet it isn't.