Turning my PC into a decent gaming rig, suggestions?

I was going to start selling my other laptop and my xbox one and try to save up for a whole new gaming pc to build, but someone told me I should just put a good graphics card in my desktop I already have ( a lenovo k450e i5 with 8gb ram and 1tb of hd but no SSD) that me and my dad use for simple stuff like YouTube and school stuff. I figured it'd be a lot cheaper and a pretty good idea, I was thinking of just putting a new around 250$ nvidia card in there and buy around 200 gba of SSD, do you think that'd be good for me just running games like h1z1 and rust at high settings? And any suggestions on any parts I should throw in as well?

You need to make sure your motherboard is compatible. If you bought some random pre-built one form bestbuy or whatever, chances are you can't do a lot with it. Even if you could, you need to consider your power supply. If it's not high enough wattage, your computer won't even turn on with the new gpu.

You have a pretty decent base in terms of general specs… An Intel core i5 is about the lowest class CPU for some games support. Your system RAM is around the recommended minimums for some higher-end games (may want to consider upgrade to 16 GB in the future, assuming your motherboard supports this) & your HDD is OK in terms of storage, but you may want to look at going with a 7200 RPM one (vs the default 5600 RPM models) for some speed improvements. SSD's have the best performance, but cost / storage / performance ratio might put that outside your budget for now.

Getting a good GPU should be your highest priority as that will seriously dictate what games you can play & what quality settings you can use. Your video card (be it NVidia OR ATI) should have at least 2 GB of video RAM, but you'll likely want to go with 4 GB if possible… But don't blow your full budget based on video RAM as chipset is important.

You will also want to check your power supply & power load BEFORE you purchasing any components since prebuilt systems (especially low-end budget models) may not have a lot of available capacity to support a power-hungry video card… So bear this in mind when shopping.

Hope this helps!