1. The case: Buy the Antec 300 as suggested by someone else as well. I have one, it's cheap yet has great features, space and fan potential.
2. Overclocking ability depends on the hardware you choose and doesn't always require an expensive motherboard.
Example: My motherboard cost a whopping $65... (M3A770DE)
Currently my CPU and RAM are overclocked...
Traditional Overclocks require you to increase the bus speed to speed to up the RAM and CPU. Messing with the motherboard BUS speed isn't a good idea unless you know what you are doing and honestly I don't know enough...
HOWEVER....
Modern overclockers (like myself) have found it much easier to buy a CPU with an unlocked multiplier. The unlocked multiplier is a quick easy setting change that increases the frequency of the CPU without needing additional changes to the motherboard.
Now... my ram.. is only kinda overclocked...
My ram is rated for 1600 MHz but my motherboard says that speed is only possible by overclocking and that it would never detect 1600MHz memory at full speed. Well, once again overclocking is getting much easier than it was in the past, the BIOS settings allowed me to manually set the memory timings and frequency without making changes that effect my motherboard. Officially for my motherboard the 1600MHz setting is "overclocked" because that chipset does offer RAM compatibility at that speed.
Bottom Line: Most people don't know what to look for in a motherboard. An inexpensive motherboard is nothing to be afraid of but I wouldn't get one made by a fairly unknown/generic brand as they could use cheap capacitors and it could kill your board early (even if you don't overclock)
Mine is ASROCK...
Other worthy brands: ASUS, MSI, Gigabyte, Intel, Leadtek, PNY
3. You won't need the thermal paste because your CPU fan/heat sync will come with some.
I would save up your money as it would not take much more to build a computer that is significantly faster.
If you saved a bit more:
choose a Core i5 CPU
choose a better GPU
Example:
http://pcpartpicker.com/p/4fFByc
My example is only $80 more but the performance difference would make it feel like hundreds more.
However, I know, saving up more money for a PC with a higher price isn't what you planned.
http://pcpartpicker.com/p/zH7kbv
This example uses the intel boxed cooler, no thermal paste, the case i mentioned, cheaper Patriot memory and a cheaper motherboard.