Your old quad-core Harpertown processor
is probably not doing too badly, especially if you've overclocked it by 20% to 3.4 GHz by raising your memory speed from 667 MHz to 800 MHz.
Would your old motherboard support two 4 GiB DIMMs, or is it limited to four 2 GiB DIMMs, maximum?
You'd be looking at $105
for each 4 GiB DIMM of PC2-6400 memory or $100
for 2x2 GiB of PC2-8500 if you ordered it from the 'egg. If you're certain that this is all
that's wrong with your system, there's no better bang for the buck than fixing what you have for $100.
I would encourage you to make the LGA1150 CPU + Z97 motherboard upgrade that you described in option 2.$70
Intel Pentium G3258$115½ -20 combo
MSI Z97M-G43 motherboard uses a Realtek 8111E ethernet PHY.$165
Asus Z97 Gryphon motherboard uses an Intel i218V.$40
4 GiB PC3-12800 Crucial Ballistix BLT4G3D1608DT1TX0 (DDR3-1600, 8-8-8-24, 1.5 V)$46
4 GiB PC3-12800 Crucial Ballistix Sport BLS4G3D1609ES2LX0 (DDR3-1600, 9-9-9-24, 1.35 V, low profile)$43
4 GiB PC3-14900 G.Skill Ripjaws X F3-14900CL9S-4GBXL (DDR3-1866, 9-10-9-28, 1.5 V)$50
4 GiB PC3-14900 Crucial Ballistix BLT4G3D1869DT1TX0 (DDR3-1866, 9-9-9-27, 1.5 V)