If you need an OS and a monitor, a budget dell on sale or a craigslist deal would be your biggest bang for your buck. Used non proprietary would be better than a new or used Dell proprietary though. But if you OS is new enough (Windows XP or newer, Windows 7 preferably) and oyu have a monitor you are happy with, put a new MB, CPU, and RAM in that old box and enjoy!
CPU: Get a Dual Core CPU, you want the fastest per core speed you can get. i3 is a great choice!
RAM: 16GB is overkill for most. Even 8GB isn't used by most and would prove PLENTY sufficient. We run a desktop (in a VM) with photoshop, and autocad architecture, virtual servers and a host server all in one box at the same time with 8GB RAM.
MB: Get an ATX board, NOT micro ATX If will have more room to upgrade when prices for memory and such come down. 4 RAM slots, more PCIe slots, etc.
Other parts you might want
Hard Drive: search HLHX or HLFS on ebay and get a fast 10,000 PRM drive for like $35.
Video: Just make sure you get a MB and CPU that have graphics on the CPU.
Power Supply: If it has a 24 pin connector, an extra 2x2 pin connector for the CPU, and ANY SATA connectors you are probably good. If you are missing several of these things, you might want a ANTEC or seasonic 350W + power supply due to the current balance being incorrect for todays setups. This is important because it would power on and could still be a bad balance causing possible errors under load or even damage.
As for OS, the recommended thing to do would be to install your OS from scratch. But, if you can't or don't want to, find an article on how to replace a motherboard. It should run you through prepping windows for an upgrade like you are talking by removing device drivers and such.