If this is your mainboard
The mainboard is compatible with Intel CPU Socket 775, Socket 775 CPU is: Core 2 Duo (Dual core CPU), Core 2 Quad (Quad Core CPU, more core is better usually), these CPU are belong to previous generation and shouln't be expensive, provided if they are still available for purchasing that is. The CPU is unfortunately not available in my region. But IMO it shouldn't be more than 100 USD fore a Core 2 Duo
The mainboard is using Intel Chipset G41, meaning it has in video processor intergrated into it (GMA X4500 most likely), though, I strongly recommend you to invest in a dediated graphic card since the intgrated video processor is out of date, and can't do anything but only basic task. For the cheap but effective graphic card, I recommend you looking at the AMD Radeon HD5770 line up, or HD5670 (ASUS manufacture recommended) line up depending on the price and your budget. The 5770 cost in my region is 80$ USD
Although this mainboard is compatible with both DDR2 and DDR3, I strongly suggest to stick with DDR2 since Chipset G41 doesn't natively support DDR3, it's waste of money since you cannot use up all the power and potential of the DDR3.
Note: in some region DDR3 is actually cheaper than DDR2, in that case, consider the DDR3, purely value wise. For me, 8GB of DDR3 costs 60$ USD. DDR2 is no longer available in my region.
Key/Mouse shouldn't cost more than 20$ both of them, quality PSU run to 50$ (500W), and you want high quality PSU, as bad one will kill your PC or slowly killing it.
Monitor, heh, 20" is about 150$ LCD I believe.
