Dont buy anything just yet, i'll pour over details and find you the best options for diff price ranges

You are limited by space (SFF) and power (240w / 25w pcie) thus the AMD Radeon HD 6570 is out of the question, even the GT730 i linked earlier is on the raggedy edge of what your pc will support, but it will at least work.
Without a gpu installed youre stuck with the onboard Intel GMA4500 which is lacking on features badly, shader models, dx support and so on, old games will run but many will refuse to even launch because of the missing feature sets.

Get the gt730, and you'll be able to run a much bigger range of games, all the way up to gta5 and watchdogs, you'll have to knock the settings back a fair degree to get decent framerates but they will at least be playable.
The gpu will also make applications like VLC, itunes and similar perform better with reduced load on cpu, also opens up the capacity for running emulators of more revent systems.

For OS you want something with minimal overhead to squeeze the most out of the hardware, so don't go anything newer than windows 7, if you go for XP, be sure to install a virus scanner and third party firewall *BEFORE* connecting it to the net!

If you go for win7 turn off aero and run a more basic theme with the bells and whistles turned off, use msconfig to disable services and applications running in the background that you dont need.

Use a legacy version of firefox or chrome , both of which while less secure have a much smaller resource requirement.

If using something newer than winxp, install dosbox to run older games that usually don't work natively, or consider dual booting win7 and an older OS like win98 specifically for running older games, just do not connect the win98 install to the net.

If you do dual boot, create three partitions, for win7 around 20gb, for xp or older, 10gb, the the third give the remaining space to, use the third for downloads , installers and user files, so you have easy acess to them from both OS's.

Last but not least, if someone gives you shit for not having an insanely powerful rig, tell them fuck you.

For the record im giving you this advice as someone running three GTX980ti's on three 2560x1440 monitors, but it makes zero difference, you don't need a high end pc to be a pc gamer and you'll find it's usually the ones trying to justify thier expensive spending habbits claiming otherwise.

Welcome to pc gaming!

Duude.. what do you do?

You're awesome.  You know that?  You're awesome. :)

Pretty good deal. Just make sure to delete System 32 to make it run real smooth.

Get a cheap graphics card and play some classic games that weren't on consoles or shitty on consoles. Games from the "golden age" before DRM and the simplification of gameplay for console audiences.

I'm thinking of stuff like Max Payne 1+2, Quake 1-3, No One Lives Forever 1+2, Deus Ex, System Shock 2, Thief 1+2 and the classic Bioware and Black Isle Studios games.

I believe she is a developer, so...computery stuff!

good call