The link that you posted shows a range of possible specs for your current computer. So even if you have the best components possible within that list, that means your processor is 7 years old. I suggest buying a new motherboard, processor, RAM, and an SSD. If your budget allows, get a new power supply also. Those can go bad after a number of years (their wattage declines over time), and the longest I would consider keeping one is five years.
You currently have an LGA 775 socket motherboard. Should you want an Intel processor you would need a newer LGA 1155 motherboard, which is not the newest socket, but the LGA 1150, which is the newest, has not yet released a CPU that is within your budget (typically they release the expensive stuff first).
However, an AMD processor would probably give you a better value, since you can get a quad core AMD processor for about $100 whereas the price threshold for a quad core Intel CPU is about $170.
This $100 FM2 socket (the newest AMD socket) A8-5600K Trinity 3.6GHz is probably the best fit for your budget. It has good graphics capabilities also.
http://www.newegg.com/Product/Product.aspx?Item=N82E16819113281
This is a matching Micro ATX form factor motherboard for under $50 that should fit in your case ():
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130661
Here is a $33 4GB DDR3 1600Mhz ram stick that is supported by that motherboard:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231423
That puts us at $180 and we've replaced your motherboard, CPU, and RAM. At this point, I think it would be wise to get a new power supply and an SSD. You could add the SSD as the drive that holds the operating system and keep your current HDD, attach it to the new motherboard, and use it as storage.
This $100 120GB Samsung SSD offers the best value right now and will do the most to improve your system's every day performance.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820147188
This $40 300W Seasonic Power Supply is a high quality, well-reviewed unit that would enable you to add a small discrete graphics card if you ever desired to do so.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151090
That puts us at $320!