Using onboard Nvidia 9100 WITH a Nvidia card

I was ripped off if there isn't  

There was another HP in a box for 100 bucks less that had a card (some low end ATI HD 3xxx) and quad core, but only 4 gig mem... I got dual core, 8gig mem, onboard Nvidia 9100... those were the only differences...... but mine was 100 bucks more!? why?

i got HP p6116f...

Why indeed.
More details please. Make and model numbers always help.

And the model of the $100 less PC?

pxxxxf with ati 3xxx HD, 4gig mem, quad core.

is there a card that will work with onboard Nvidia 9100???

pxxxxf
LOL

This maybe one of those 'slap the head, I coulda had a quad core and save a $100' moments.

No.

Hybrid SLI is not an option on that Pegatron M2N78-LA motherboard.

source: HP website

Luckily the motherboard does have a PCI-e x16 slot so you can add in a video card.
With that 300W power supply you should be able to use a Radeon HD 4670 or GeForce 9600 GSO or 9600GT.
Best Graphics Cards For The Money: November '09
If you don't mind pushing your PSU a little bit harder you could step up to the GeForce 9800 GT or Radeon HD 4830.