The Core 2 Duo E8400 is an LGA775 CPU, the motherboard is an LGA 1151 CPU, so no, not compatible. You won't find any CPU for LGA 1151 for only $25, it's a brand new socket, and the low end dual core models aren't even out for it yet. The cheapest CPU available for LGA 1151 right now would be the i3 6100, and that will cost you about $100 if you can find one.
For a new CPU you're looking at a minimum of about $70 for one, and that's if you go for the AMD Athlon x4 860k, which is about the cheapest CPU you can find that meets the game's minimum requirements. You'd have to get an FM2+ motherboard instead and swap out the DDR4 RAM for DDR3. You're not going to get anything cheaper that would work unless you're willing to look at used hardware.
Thanks, this is what i've found
GIGABYTE GV-N75TOC-2GI G-SYNC Support GeForce GTX 750 Ti 2GB 128-Bit GDDR5 PCI Express 3.0 Video
129.99
$20.00 Mail-in Rebate Card
G.SKILL Ripjaws 4 Series 8GB (2 x 4GB) 288-Pin DDR4 SDRAM DDR4 2133 (PC4 17000) Memory Kit Model F4-2133C15D-8GRR
$48.99
MSI B150 PC Mate LGA 1151 Intel B150 HDMI SATA 6Gb/s USB 3.1 ATX Intel Motherboard
$89.99
Intel Core i3-6100 3M 3.7 GHz LGA 1151 BX80662I36100 Desktop Processor
$124.99
Dynatron K5 92mm 2 Ball CPU Cooler for Intel LGA Socket 1151 / 1150 / 1155 / 1156
Will these items, meet fallout4 requirements?
I used a website, to check out if everything was compatible, I'm gonna use my same case, with my old hard drive, and power supply. I've gone with another atx motherboard, so it'd fit right in my case i have now. What i plan to do with my old parts, is buy another case, hard drive, power supply to put all my old parts into. That way i can take my parts, and give my girlfriend a better computer to use than her laptop.
Thank you