That won't be easy to do since the 2.1 system gets its power from the subwoofer module. You'd have to either go with a passive speaker pack with a receiver or spend a premium on active monitors. I don't think you will get anything decent for $300. Cheapest would be the Energy Take 5.0 speaker package plus a refurbished receiver for maybe $350 to $400 and use the existing ProMedia subwoofer. Klipsch does have their HD Theater speaker packages starting at $400.
HD-DVD = 94
Blu-Ray = 120 ( 24 Warner red2blu )