Ok i can give you some suggestions, first your cpu and memory look good with only the video card lacking.
An important note: before buying a new video card make sure your psu can support it and that i can fit in the case. i wasnt able to find what your psu is and the case looks small, most modern cards are quite large.
the gts 250 is a rebranded 9800gtx+ which means you will only get a small improvement over your current card. The gtx260 would give u a good boost but still not huge. You might want to consider a gtx260 core 216 or gtx275, these will give u close to 2x - 2.5x the performance. Also remember that more memory doesnt always mean better performance.