Spend as much as you can afford, it's worth it, but for reviews you should get a couple of the "Home Cinema" magazines, they normally know what they're talking about, then search the net or try Richer Sounds for the best deals around.
I have a set of B&W VM1s and an AS1 Subwoofer running through a Yamaha amplifier with a chipped Toshiba 220 DVD all shown on a Toshiba 32DZ26, I like it!
One important bit of advice, whatever you buy, spend a decent amount on new leads for everything, makes a significant difference to the sound and picture quality.
You wanna be careful the model you've chosen has a SCART output - not all Sony all-in-one packages have them.
You also get an FM radio built in which is pointless if you already have a tuner.
Also, some subwoofers are active and some are passive depending on the power output I think. Some satellite speakers in the low end Sony systems are poor ( build and sound wise ).
I would go for a seperate DVD, home cinema amp and surround speaker package ( based upon your existing hi-fi speakers if you can ). Better in the long run, although possible more expensive ( but not if you already have the basic stereo pair ).
Remember the rear speakers should be di-pole models to achieve a proper ' surround ' effect, ie. don't think you can get away with another ordinary stereo pair of speaker for the rear.
Take your ears to a proper hi-fi shop ( Jeffries, Powerplant or Sevenoaks ) for a test drive.
My personal home cinema choice would be the NAD S170i surround sound pre-amp and S 250 5-channel power amplifier but at £ 2999 for the pre-amp and £ 1999 for the power amp I can't afford it !