Good mobo for a e5200

Newegg had one msi p35 atx board for $49.99, but I don't know if they have any left. Otherwise, checkout some g31 boards by downloading the manual before ordering. Check the bios for voltage and memory adjustments.

If you can find it now: The P45-DS3L or the P35-DS3L were good for OCing that chip. They don't make the P35 any more, but you may be able to find it online for ~$50-55.

If budget isn't that big this is what I would get:
http://www.newegg.com/Product/Prod [...] 6813128357
It's got a good basic set of OCing options (manual CPU/RAM voltage, manual FSB,etc) and it's only $53

Note: I didn't read through the entire manual, I just skipped ahead to the MIT section where the OCing options are listed, and it didn't display RAM timing functions, how ever, I believe these are available by pressing <F1> like on most Gigabyte boards. Linky to manual: http://asia.giga-byte.com/FileList [...] s2c)_e.pdf

If you need OCing help, check out the P35-DS3L OCing guide available on the forum here.
Only glaring limitation I see is that it only has 2 slots for RAM. Then again, 4GB (2*2GB) is enough for an average user.

Nope.

^Wich motherboard? The G31M-ES2L?

You can use it with the Antec 300, BUT you will need to move the stand off screws and/or you will not be able to putt all stand offs in.

^No. It only has 2 fan headers on the board. However, you can get a fan controller to fir the other fans in,BUT 3+ fan controllers are pretty expensive (~$30 for a good one) in this case you might as well get a better board. Or you can get a splitter: http://www.newegg.com/Product/Prod [...] to%20molex
$4

^Yes, in theory.

Note: I have never used anything like that before, so I can't really comment on how it'll work out.