If you don`t know which one you own ... then probably you use onboard sound card which isn`t decent :(
It`s the best to plug SP2500 with 2 x RCA male cable:
http://ucables.com[...]
I`ve bought ASUS Xonar Essence STX (STX is a PCI-E sound card, ST version for PCI) which also use 2 x RCA so they do fit together.
I`m not very familiar with sound cards ... just bought the one from premium level as ... i always say that quality costs ;)
Maybe you could find some decent sound card with 2 x male RCA input so you fit in 2k budget.
But first check the sound card? - just take a look at the back of your case ... or open the case to see if there`s any external sound card installed.
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P183 V3 @ 300rpm, P8P67 Deluxe, i7-2600K @ 4GHz, NH-D14 @ 500rpm, C300 128GB, GTX 580, AX850
G:9x/15/25/35, Xonar STX + SP2500, BenQ 120Hz