1. If you spend $20k on any PC, you're a fool.
2. If you do so at this time of year, on the eve of Haswell and new GPU series being released, you're twice a fool.
If you actually have $20k to spend on PCs, break it into 10 $2000 chunks and buy yourself a high-end machine every two years. That way in six years when your $20k machine would be losing benchmarks to off-the-rack devices, you can be on the cutting edge.