Glad you have some kind of discount available, because I can list a dozen sources that have those exact cans available for $100 cheaper.
Anyways, HD558's are great given the price and the potential for the "Foam Mod" which you can find more details on if you google 558 Foam Mod. However, they are a bit light on bass. If you consider yourself a "bass head" you'll be disappointed in the capability of the HD558's.
I assume you mean
PCI-E x1 slot. That gives you plenty of options for sound cards.
For sound card, pick a
Xonar DGX which is the cheaper, sports a built-in amp and provides you with all you need for headphones. Regardless of what headphones you opt for, this is the sound card you will want given your budget. Feel free to shop around for some kind of open box deal or refurbed card. You won't miss out compared to buying a brand new card except for the higher price tag.
For headphones,
Audiotechnica A700x will perform well while having bass available. You could go for the open-ear
AD700x, which have decent bass but the A700x definitely wins as far as bass is concerned. As the owner of a pair of AD900x, the wing design utilized for all audiotechnica's audiophile headphones is unbelievably comfortable even during long gaming sessions. Then again, if you have a smaller head the wing design might cause them to sit lower on your head. This is something to consider, unless you have a large head as I do.
You could shop around for some
Ultrasone Pro 550's which are definitely bass heavy, if that is what you want. Though I've read a lot of mixed reviews. One thing is for sure, folks either outright love them or outright hate them.