PC advice

So someone I know is selling me a computer for $500. He's trustworthy. Here's the setup:

Core i5 3570k
ASUS Z77 Sabertooth
16GB (8GBx2) Corsair Vengance
NZXT Switch 810
HIS Radeon HD7870 GHz Edition
XFX Core Edition 850w 80+ Bronze

Here's what I currently have:

Phenom II 965 (x4)

Gigabyte GA-770T-USB3

8GB DDR3 1600

Full size case with 5 120mm fans

PowerColor Radeon HD 6850 1GB

600W PSU Offbrand

Should I go through with the deal? Will the i5 3570K hold up for a few more years or is it already obsolete? Any advice would be appreciated.

3570 is Ivy Bridge, and is 2 generations behind the "current" setup with Broadwell. 

This should give you some ideas about comparisons between the CPUs and video cards:

http://cpuboss.com/cpus/Intel-Core-i5-3570K-vs-AMD-Phenom-II-X4-965

http://gpuboss.com/gpus/Radeon-HD-7870-vs-Radeon-HD-6850

That's a bit of an upgrade between your current system and the one your friend might sell you.

Case looks good on paper, though is only so-so:

http://www.anandtech.com/show/5479/nzxt-switch-810-when-too-much-isnt-enough

Pretty good motherboard:

http://www.tweaktown.com/reviews/4878/asus_sabertooth_z77_intel_z77_motherboard_review/index2.html

Looks like a good PSU as well:

http://www.newegg.com/Product/Product.aspx?Item=N82E16817207011

Overall, it's a reasonable deal IMO.  I wouldn't just take my opinion on the deal, of course.

No. The biggest question to ask yourself is what do you do on your pc?

If your into gaming you'd be better off buying a new video card (if that's why you think your pc is getting slower). You still have a decently fast quad core cpu (granted its older). But gaming is more towards the gpu then cpu today.