Yep, ExtremePC is right. Your issue is principally in the networking aspect.
If you don't have the option to hardwire, you may need to consider draft 'n' wireless for both throughput and range.
Another option to consider is the new "200Mbs" powerline networking solution from Netgear. See the thread below for more info:
http://www.xpmediacentre.com.au/comm...light=amilwain
Both options are pretty new to the market and have mixed reviews and varied degrees of success dependent largely on the operating environment.
If you decide to go with draft 802.11n, there are some new options coming to the market with both 2.4 and 5Mhz bands (sometimes referred to as dual-pipe technology) for optimal performance and signal robustness. Some manufacturers guarantee compliance with the final 802.11n specification also.
Whichever way yo go, do plenty of research.
Also with regards to wireless PCI cards, often they give poorer performance unless they have an external aerial on a lead (ie. not the stubby arial which comes directly off the card itself).
Metal HTPC cases, AV racks and other AV devices act as great blockers of WiFi signals, so it's best to get the aerial as for from them as possible.
Often wireless bridges work much better for that very reason.
Bottom line though, even the best 54Mbs signal ain't gonna cut it for reliable HD streaming I'm afraid.