You will get many opinions on this topic. My personal choice would be to use onboard, but I am not a gamer. Since your MB/CPU is not the latest and greatest, you might see some real benefit to releasing the CPU cycles needed for the onboard sound controller and handing that chore off to the sound card.
I would try benchmarking with/without card to help make the decision.
Finally, what do your ears say? How does the sound compare? I bought a SB Live! card to attempt to eliminate the background noise, still there, so off to eBay it went. Xfi and e-Mu offer better components, but as you said that is not an option.
Sound is highly subjective; technically, the onboard is more advanced, and may offer better sound, but it has to deal with the noise from proximity to the other components and costs CPU cycles to run...your choice should be based upon your needs and ears.
Tony