The PXI-2595 should be just fine as a replacement. It has a higher bandwidth (DC to 5 GHz vs. DC to 4 GHz), the same max voltage (30 VDC), higher max current (500 mA vs. 330 mA), and is also 4x1 50 ohm coaxial.
The only possible problem is that the scan rate for the PXI-2595 is slightly slower than that of the PXI-2591 (22 ms vs. 15 ms), but this might not be a problem for you if you're not already maxing out the speed of the PXI-2591.
