if you're struggling at 640x480, you are gonna have to get yourself a new card... i know outrun on pc needs at least a FX5600 to even run at potato settings, a geforce 6200 is recommended. quite honestly, unless you are running a 15k monitor or for some reason want to run native 15k resolutions and junk and need the AMD card with custom bios to lock 15k what have you, here is really no reason to have an old video card as almost every modern video card supports 640x480. i think the 950 GTX was probably the last videocard to feature analog VGA outputs before everything went digital with DVI-d, HDMI or Displayport. check to be sure it has a DVI-A port and not DVI-D cause you cant use an adapter to convert it to vga on DVI-D
most 31k monitors are smart enough to ignore anything other than 31khz (or whatever other modes it supports like 15 or 25) so even if you boot in 800x600, it will probably come up scrambled... but once you enter you FE (or whatever) and switch to 640x480, its fine.