Your best bet is to buy the Z-5500, its your cheapest alternative. Dedicated AV receivers and home cinema speakers will get far too expensive and can be complicated to set up. You'd be limited to 5.1 sound only with the Z-5500
It can be purchased here--->
http://www.newegg.com/Product/Product.aspx?Item=N82E16836121120&Tpk=z-5500
You would have to connect the Z-5500 to your computer as normal with the Prologic feature enabled, however the PS3 will be connected using a fibre optic cable to the Z-5500. PS3 games support Dolby 5.1 so you'll get surround because of the Z-5500s built in decoder.
I'll try to explain the buzz words:
Decoder - Decodes the existing pre-encoded Dolby bit streams
(PS3 games are Dolby pre-encoded)
Dolby Digital - Dolby's algorithm for 5.1 surround sound. (needs coaxial or fibre)
- PS3 games are encoded in Dolby 5.1 by default so the Z-5500s receiver can decodes the streams.
Prologic - Dolby's up mixing algorithm to convert stereo into 5.1 or 7.1
- You may need this feature enabled as your PC may only output stereo initially.
(hence the problem you are having currently with your creative t7900!)