PSU: How many Amps are on the +12V rail?
My computer at work uses 375W Logisys PSU with 16A on the +12V rail (16A x 12V = 192W). Total wattage is typically about 20 to 40W more than +12V wattage on good PSUs, that would put this "375W" at 220W. So I'm guessing yours is a 200W PSU.
Good overclocking basics:
http://www.overclockers.com/forums/showthread.php?t=263753
A 6850 at 1024x768 is kinda pointless--I really think you should upgrade your monitor to a 22" 1920x1080 or at least 21" 1680x1050 monitor. Your money would be much better spent on a US$80 5770. But like I said earlier, if you've got US$225 to spend, you should get a US$65 5670 and a US$130 22" monitor.
Do not confuse an Athlon II with an Athlon. There is no Athlon x3. You currently have an Athlon II x2. That would be an Athlon II x3. The upgrade would provide a minimal performance increase, but be a big waste of cash. If you're upgrading your CPU, try to find a Phenom II x3 720BE for less than US$75. A new AMD processor is coming out very soon, so I strongly advise you not to upgrade your CPU until that one comes out so that prices drop as lots of people sell off their old CPUs to upgrade.
If you want your graphics card to go a long ways for years to come, your budget is pretty small...but it's your money. Personally, I think a decent monitor does a lot more for day-to-day satisfaction with a computer than a great video card compared to a good one.
All this talk of what to buy depends on where you're buying from and how much parts are. Do you buy from a retail store or online? Online is typically cheaper. You are in the Philippines, right? Are you buying there or buying through a relative in the states who ships stuff to you? Do you have any friends/relatives maybe somewhere like Guam that would bring stuff back to you? I don't know what your system is, but let me know how you're buying so I can figure out what's in your budget and what parts are available.