I have an old desktop that was built by a local computer place that I was unhappy with since day 1. Should have researched components and built it myself. It has always been unstable (with all operating systems that I've had on it). Since it's almost 7 years old and was built with not totally current components at the time, it's pretty slow too.

Here is the current hardware list:

Antec 300 ATX Mid Tower Case
Antec EarthWatts 650 power supply
MSI P55-CD53 motherboard
Intel Core i5-750 processor @ 2.66 GHz
320 GB Seagate 7200 RPM hard drive (SATA2)
1 TB Western Digital Blue hard drive (added in 2015)
4 GB of Kingston RAM (1600 MHz DDR3 PC312800 SDRAM)
MSI GeForce GT240 graphics card
Samsung 22x DVD R DVD Burner with LightScribe (SATA)
Rosewill 802.11 b/g PCI Express Wireless card

I'm considering giving it a cheap makeover. My priorities are:

1. Cost (less than $500 for sure and cheaper is better)
2. Reliability
3. Friendly to Linux
4. Continues to support dual screens

It is primarily used for bookwork (Quickbooks and spreadsheets). I just want it to do so quickly and reliably. I am thinking if I replace the motherboard, processor and RAM I can surely have a pretty decent, reliable machine.

Most places I search on the internet are geared towards gaming machines. I just want reliability and I want to be able to run Linux. Anyone on here have enough experience to make some suggestions? Thanks.