Will it work, in XP most likely with no problems. In Vista (after the platform update that added DirectX 11) and Windows 7, probably not. Reason for saying that is you are mixing two different generations of card that use a different driver
model. The 7800 GT will use the WDDM 1.0 driver model and the 9500 GT uses the newer WDDM 1.1. Both cards may install from the same driver package, but they won't use the same driver.
The problem is that when you mix two cards from the same manufacturer (ATI with ATI, Nvidia with Nvidia) that use the different driver model, the older card cancels out the newer and as a result the newer card doesn't work at all.
For what you're looking to do it might be a better idea to grab something like this, Newegg.com - GIGABYTE GV-N84STC-512I Rev2.0 GeForce 8400 GS Supporting up to 512MB(128MB onboard) 64-bit GDDR3 PCI Express 2.0 x16 HDCP Ready Low Profile Ready Video Card
uses the same WDDM 1.1 as your 9500 GT. Also would use a lot less power than the older 7800 GT and produce a lot less heat, which would be even more beneficial when all it is ro be used for is extra monitors.