You don't need extra powerful hardware to do general usage stuff, much less even medium level to heavy gaming. My old Athlon X2 5000, 4GB RAM, GeForce 9800GT 1GB, and 500GB drive can effectively push many modern games using Valve's Source engine which is rather intensive for PCs. If mine works fine, this PC I'll showing you will work fine also.
At best, this
is more to about what you might need for at least the next 5+ years at best. Normally I look at units that can be upgraded, but often don't require much to upgrade them over a 5+ year period, and don't need upgrades unless absolutely necessary.
The CPU is very good, the RAM is more than adequate for anything really, you have plenty of storage space, and the GPU is powerful enough for anything. The only thing I would maybe recommend is buying a new power supply unit with at least 750 Watts or better like:
and that's it.
16GB of RAM is overkill by any standard. It's nice to have if your a tech junkie, but realistically even Windows 7 Ultimate 64-bit (which requires a good bit of power to operate) will run perfectly fine with 4~8GB or RAM. For Slackware 14.0, it'll be perfect. Nvidia might not have the most fanfare favorite for Linux, but they do support their products far better than AMD/ATI does. The GT520 1GB is good for anything really.