Technically, Sumit, you can use the Xonar D2X on your motherboard in any one of the other PCI-E slots available.
But practically, the Xonar is an inordinately long and broad card. That it requires you to connect the floppy power cables also might complicate things. Please check out the space available within your cabinet. You might be able to fit both of the cards in (the Xonar along with your existing display card that is). However if you are housing your PC in a room without AC I think there may be problems related to overheating.
On the analog outs, the Xonar gives you analog outs for all of the 7.1 channels. Goodies are one thing you cannot complain regarding the Xonar D2X card!
Further since you have probably seen me singing praises of this card, I also think you should know about the downside that I have experienced. The drivers for these cards from ASUS are among the worst I have experienced. If you are banking on ASIO output, then the default drivers provided will not give you true 32 bit ASIO. For that you would need ASIO4ALL - and yes, that will take care of your needs adequately. But then, mature drivers from compaines such as Maudio save users so much trouble. And there are regular updates too - unlike ASUS or for that matter, Creative.