Is this good for a server

Connection (Home hosted)
Download: 150mbps (Usually hangs around 80-120)
Upload: 12 mbps
Internet 10/100/1000

Computer hosted on (I got an old server from a friend for free, might as well put it to use)
4GB DDR2 (Upgrading to 8GB of DDR2)
Quad Core Xeon at 2.13 GHz Xeon-X3210 8MB Cache
320 GB of hard drive space, 2 disks raided to mirror Down the line I plan to upgrade this into a Raid 1 with 2 500GB SSD Evo)
Windows server 2012 Standard (Will having this OS help in any way, hurt in any way or do anything to the GMod server)

I plan to either run a PrisonRP or DarkRP server, not with every addon under the sun, but a few custom tools, textures, and skins/weapon packs. No or few cars. With a slot limit of 60.

Yes, it’s fine. The CPU is going to be your bottleneck assuming you don’t host the FastDL on the same node.