Refreshing an outdated PC

Simple part compatibility:

Motherboard and processor must have the same 'socket'. Examples are LGA1155, FM3, FM3+, etc. This is so the processor fits into the motherboard correctly, and the motherboard knows how to communicate with the processor (and vice versa).

The RAM must fit the motherboard (mobo, for short), but most RAM will say 'desktop' or 'laptop' ram to make it easy. Then, you have DDR2 and DDR3 ram as the most common types. Any new mobo will support DDR3 ram (and DDR2 as well), but older mobos only supported DDR2. I think DDR3 mobos are always backwards-compatible with DDR2 (aka a DDR3 mobo will always support DDR2) but don't quote me on that. Also, get 'non-ECC' ram.. ECC is for servers, which you're not building.

Video cards need to fit into a certain type of slot - PCIe, PCIe 2.0, PCIe 3.0 are the most common now. Check what slots your mobo has open, and if you have a compatible video card, then you're good. Some of the slots are backwards compatible, but check Wikipedia for your mobo's specific slots to make sure it can handle your video card. AGP is a very old standard, basically useless now, but you might occasionally see AGP cards around.

Hard drives (and SSDs) use SATA 2.0 or SATA 3.0 connections (aka 'Sata 3Gbps and Sata 6Gbps, which are the speeds that the 2.0 and 3.0 standards can handle, at maximum). I'm pretty sure they're backwards compatible, so you can use a 2.0 drive with a 3.0 connector (and vice versa) but you'll only get 2.0 speeds. Hard drives and SSDs also come in different sizes(which only matters for compatibility with your case), where 3.5" is the 'desktop' hard drive size, and 2.5" is the 'laptop' size. SSDs are usually 2.5", but most come with mounting kits so you can fit it into a 3.5" slot in your case (and most cases now have 2.5" slots for SSDs). I think some SSDs are 7mm, but generally those are more expensive. Don't quote me on that last part though, I could be crazy.


Other things to keep in mind-
1. Video card is the most important thing for games and most other graphics-intensive tasks. An i3 processor is enough power for gaming as long as you have a good video card.
2. Ram speed is not that important. DDR3 is a worthwhile boost over DDR2 ram, but don't pay more for faster DDR3, as it will not be a noticeable difference, and that money is better spent going toward a better processor or video card.
3. 8gb of ram is enough for most people. Unless you realllllly have a lot of junk open at once, or multiple graphics-intensive games, or do a ridiculous amount of video editing or something, 8gb should be fine. Then again, ram is cheap, so if you think you might want more than 8, 16 isn't a budget-buster. But you'll only notice that extra ram if you 'saturate' your current ram by having a lot of stuff open or programs that take up a whole lot of memory to run.
4. I'm tired, hope that helps .