What Else Do I Need To Take Full Advantage Of The Logitech Z-5500 5.1 Sound System?

Unless it's an nForce 2 chipset (which the P4 does not have) the digital only does 5.1 from DTS/DD sources (DVDs). In order to get 5.1 from games, you need to use the analog outputs (Front L/R, Rear L/R and Sub/Centre). I can't find a decent picture or information as to whether or not you can use that.

You know the headphone jacks right? you need 3 of those to output 5.1 in analog. Most soundcards, esp. on board ones, have connector configuration, so you can configure the mic in and line in to be centre/sub and Rear L/R

Sounds good to me.

Like I said, that only provides 5.1 in DTS/DD streams, like in DVDs. Everything else will be in Stereo. I have that problem.

The only reason I can see to use digital is if your amp doesn't support 5.1 analog.... like mine, or you like to use 1 cable instead of 3/6. My soundstorm was perfect with it. DD 5.1 through optical.
Now I'm stuck with 2 channel audio unless I watch an AC3/DTS(/OGM?) encoded file. ProLogic is a nice idea, but I find it tends to wash out the bass and focus too much in the centre channel. DD is far far better.

>realtek onboard sound have config for u to change the line out and mic into center/sub and rear.

Like I said, most sound cards do.

>cuz i didnt know that soundstorm doesnt allow for 5.1 sound when gaming thru the digital out.

The soundstorm does support 5.1 in games via digital out. It's the only card which does.

>you said no onboard did Realtek is onboard not a sound card. i was mainy correcting u saying that no onboard did.

I never said no onboard did. You are misreading my post. I said most cards, esp. [<Especialy] onboard cards have this feature.

The soundstorm (boards with the MCP-T, not nVidia boards with generic on board sound) is the only card (to my knowledge) that does the on the fly DD encoding which is needed to get 5.1 sound from a digital cable in games.
Every other card will output in stereo accross a coax or optical cable unless an AC3/DTS stream is passed through.
Creative cards from the Live 5.1 and above have a seperate custom digital output which will do 5.1 etc, but only when used with Creative speakers.
Basicaly:
If you have a soundstorm and Digital speakers, you can use the digital cable to get 5.1 in games.
If you have any other card, the digital cable will output stereo in games.
If you have a creative card and creative speakers, you can use the custom digital cable to get 5.1 in games.

Indeed