Sound Card Upgrade Advise

Hi there everyone, this be my first post here so go easy if any silly questions.

Im looking at installing a new sound card for my Lenovo Thinkcentre m52 8213 Pc running win7. The PC is used primarily as a media centre hooked up to 40" Sony Bravia LCD. And at the moment an Onkyo TX-SR805 7.1 Receiver via RCA leads to PC headpone jack for sound (i know this is a bad connection). Im looking for a card that will give 5.1 or 7.1 surround to the amp, and subwoofer and CD plus quality from itunes to my speakers. At the moment when playing via Pc the sound is very muted and i need to crank the amp to get real volume, also the auto detect subwoofer lights up but doesn't respond to increase in db.
When playing straight from my iphone to the amp with same leads though(no pc), the sound is a big improvement with volume and i can get awesome bass from my powered sub.

Any recommendations? I'm assuming optical out would be best for hifi or hdmi?

Cheers, and thanks in advance for any help

yeah maybe a bit too expensive, say $150 or less. I couldn't figure the connections at the back, is that Dvi? headphone and mic inputs. but no rca, optical or hdmi? ideally i wants to use the best possilbe sound connecter to the amp for 7.1

The Dell 8400 requires a PCI sound card (I do a lot of work on the Dell users forum). I have an HT Omega "Striker 7.1" card. It works well in Windows 7, both 32 bit and 64 bit OS. It's available at newegg.com. One note, whatever add on sound card you get, you WILL lose the front panel headphone and mic capability due to Dell's proprietary and undocumented front panel cabling.

Newegg.com - HT | OMEGA STRIKER 7.1 Channels PCI Interface Sound Card

The card upgrade will be used only on my lenovo thinkcentre m52 8213 pc. Im assuming that because i have 7.1 amp with 7.1 hifi speakers hooked up already, i would just need one connection (from sound card to amp) via maybe optical and the onkyo amp should do all the work. Its seems you can get very good cards at a very good prices I was expecting to see some very expensive units