In order to help you out the exact model number of your system is required.

JOcean is correct. If we have to guess the X-Fi and gtx 260 are original Dell oem pci sound card and PCIe gen 2 video card used in a number of old XPS models such as 435T, 720, 730, 8000 etc.  We can surmise the cpu is intel 1st gen Bloomfield or lynnfield or older cpu.  Senior community advisors on this forum would recommend not to invest in gpu upgrade in these old models and to save funds for a more modern platform.  But just for discussion, 1st gen intel cpu can be paired w up to gtx 770 without bottleneck.  A gtx 970 would be overkill as it is too fast for cpu.  GTX 770 would require a psu upgrade as it is power hungry.  Lower power and spec upgrade includes gtx 960, 760, 660; if you get a gtx 680 that may do too although it is also high power.  An AMD R9 290 or 280x card comparable to gtx 770 would do too although AMD cards in general require more power.  Those old Dell XPS cases are not known for good air flow, so case mod to add more fans is needed for high power gpu.