Well the Core 2 Duo 8600 is probably irrelevant, or at least surpassed by any Core i3 (2 x 4 thread) CPU. The Q9650 is the obvious choice.
The GT-1030 is only going to be best at 720p gaming, certainly with respect to newer titles.
As for the "405 watt PSU", I guess that's going to depend on exactly what brand it is.
P-43 / P-45 boards are going to be hard to come by new. (The word "used" always scares the crap out of me).
Memory is sky high right now, at least for DDR-4. Don't know about the prices of DDR-3 ATM, though.
I'm not sure your definition of relevant exactly meshes with mine. I'm using a G-41 with a Pentium E-6300 (No, not the original Core 2 duo), with 4GB of RAM, and 32 bit Win 7 Home. Serves me fine for the web.
You're actually going to have to be a bit m,ore specific about your needs, as "relevant", is subject to a wide variety of interpretations.
You're certainly not going to conquer Steam gaming with an E8600 and either of those video cards you mentioned.