If you are not a gamer, all you need is fast 64 bit CPU, lots of RAM, and fast hard drive for PixInsight processing. You just need high computational power. I don't think you don't need high end video card. My video card is Sapphire 7770 and costs only $160. My new complete system costs me about $1500 to build excluding the monitor which I already have. This includes i7-3820 LGA 2011 CPU, CPU fan, ASUS motherboard, 16GB RAM, 1TB HDD, new DVD burner, 1KW power supply, Antec DF-85 case. Two most expensive parts are CPU and motherboard and they costs about $600. Software of course is extra including PixInsight.
Peter