Given how expensive those speakers are you'd probably be better off just getting a second set, or a better set of speakers with multiple inputs.
Alternatively, since you don't want to play the TV and computer simultaneously you could use something like this Amazon.com: TCC TC-716 6-Way Stereo Source Selector; BLACK VERSION: Electronics
HT: Samsung LN32B460 32" LCD (greyscale calibrated) | AVR: Yamaha RX-V765 | L/R: Emp E55Ti | C: Paradigm Center 1 | Rears: Sony Junk | SW: Klipsch RW-10d miniDSP | Sources: Acer Revo Ion running XBMC with mySQL library server and 4 TB HDD, PS3, Technics SL-Q2 TT | 2x Aura Pro Bass Shakers
Bookshelf: Yamaha RX-V992 | Paradigm Mini Monitors
Shelved for now: Paradigm 11 SE Deluxe