Soundcard wise im using an Auzentech Forte... It's about 4-5 years old as well.
I think the card cost me about $250 back then, they are around $150 now i think.
The sound quality between that & onboard is really night and day as the MIC jack is actually powered by an onboard amp etc. Hence you get much better quality through headphones that actually benefit from that.
So again - must make it clear - a onboard card will ONLY give huge benefits if the headphones will benefit from being amplified with moce juice to drive them. The Senn 555's are cheap as now, so for a low-mid range solution IMO they are perfect. You will get gains on other headphones not designed to be amp'd, but I believe they are not as significant.
My 555s are modded to 595, there is a guide on the net somewhere... That said they are almost stuffed now after 5 years. Just the plastic clip/surrounds have broken and does not affect operation. I don't treat them as good as you probably should, so I'm quite impressed with how they have performed overall.