not all SIMMs have to installed in pairs, only if the SIMM has a 32-bit data path and the MOBO has a 64-bit data path, then you need both, but some older boards had 32-bit data paths and therefore only needed 1, with dual channel DDR, you have a 128-bit data path and need 2 DIMMs to fill it
as far as the memory issue youre having, i would try and put the memory in slots 0 and 2 (1&3 physically) and see if it works if not you might have a bad stick, try putting the 2nd stick in slot 0 (1st) by itself and see if it boots, if it does then the problem lies in the configuration, if not its the memory, another thing to try would be to put the old vid card in like you siad and try booting then installing the drivers for the new on and rebooting with the new one in, i hope this helps