Unfortunately, ASRock socket 775 hybrid motherboards (boards with both AGP and PCI-Ex16 graphic slots) have the PCI-Ex16 slot hardwired to run at x4 speeds.
Tom's did an article
a while back concerning limiting the bandwidth of the PCI-E slot. I'd imaging newer cards and more demanding games would only limit the card's functionality even further.
I'd recommend keeping what you have now until you can upgrade the motherboard, cpu, and graphics card all at the same time.