i was thinking about using the 550ti as a dedicated physx card is this viable or would i be better off just using a single gfx card and can my psu even power it.
Yes, it is a viable option and, yes, your PSU can power it.
if it is a good idea to have both graphics cards in my machine whats the best set up to allow for an additional sound-card because the noctua d14 is very large it blocks the first pci express slot then i have my gfx card in the first pci slot which in-turn blocks another pci express (its a 2 slot card) so i am not sure if all 3 will even fit.
Based on this MSi Z87 GD65
you have 3 PCIex16 slots, so put the 550Ti in the last (bottom) one. That should give you room for the audio card
i am looking to buy a sound-card i listen to a lot of music and of course game a lot and the on-board audio just not loud enough through Audio-Technica ATH-M50S and i am a mild audiophile all my music library is flac will i notice a significant difference getting say an ASUS Xonar Essence STX or ST in volume, clarity and bass or will i need better headphones to justify a sound-card?
Regarding the sound card, see my comment below. Regarding the headphones, someone else will have to check me on this, but most decent headphones are USB and wouldn't benefit from the sound card (I don't think the sound card has a USB port). Also, high quality headphones I believe
have built in technology for audio re-production making the sound card a moot point.
and finally regarding the mobo and the sound-card is it better to get the ASUS Xonar Essence STX PCI Express or ASUS Xonar Essence ST PCI because of the slot layout of my mobo and with future mobos in mind.
PCI slots have been replaced by the newer PCIe slots. In fact, your motherboard doesn't have a PCI slot (based on the link I have above). So, if you plan on re-using the sound card, I would recommend the PCIe version.
Hope this helps.