If you have a 2.1 set up then you probably wouldn’t give a damn about it emulating the 5.1.
On initial set up onboard is serviceable. There is a noticeable difference, even with bargain 5.1 system, but it can be added later. The GTS 250 matches up about even in performance with the 4850. The 4850 is a might bit cheaper. If you were looking for the performance of the GTS 250, and don’t need the extra the 4870 needs you could go for a 4850 even.