You have not stated whether your system has slowed down a lot or if you simply want an upgrade plan to get more out of it so I will do my best to cover both.

For a start with 3 sticks of RAM installed you are not running in dual channel mode, here is a quote from your motherboard's manual:

This should not make the system painfully slow however but having 4 identical sticks of RAM will enable dual channel and provide somewhat of a performance increase.

My own system performance when I was using windows 7 degraded badly over time, in fact windows 7 for me proved to have the worst degradation of any OS I have used, the only way I found to address this was to periodically perform backups and do fresh installations. I won't say that it is windows 7's fault or that it is a bad OS but for me, given the amount of software I install and remove from my system the degradation was pretty severe. I could go on here with dozens of things that will supposedly help improve windows performance but really, in the end the best and safest way is to perform a backup of important files and do a fresh installation. 

As time goes on the OS collects clutter, partially uninstalled programs leave entries in the registry, temporary files clog up your hard disk and windows update can tend to install a lot of things you neither asked for or wanted. Then on top of this there is mallware, viruses, addware and rootkits that can all negatively impact performance.

So if you are unwilling to perform a fresh windows installation then I suggest you download and install: 

Mallware bytes, free edition

Spybot search and destroy 2, free edition

and your choice of antivirus

These are the tools I personally use but there may be better options out there. Install and run these on your system and clean out all the unwanted viruses, spyware etc and you should see some performance return to your system. 

Another app I find useful is Piriform CCleaner, it will help to clean out redundant registry entries as well as clean out temp folders and other unwanted cached files. Be very careful with programs that claim to speed up your PC, many of them are simply shells for mallware or worse. CCleaner is a good one and I have never had a problem with it but some others are a scam, promising massive improvements if you just buy their product etc only to have it change your default search page and cause general havok with your system.

I hope these little tips are of some use to you. If you have any more specific questions please feel free to ask 

asrock h87 pro 4

24 gb ddr 3  dual channel

7.99.6 frequency

  Operating System
   Windows 7 Ultimate 64-bit SP1
  CPU
   Intel Core i5 4670 @ 3.40GHz 47 °C
   Haswell 22nm Technology
  RAM
   24.0GB Dual-Channel DDR3 @ 799MHz (10-10-10-27)
  Motherboard
   ASRock H87 Pro4 (CPUSocket) 39 °C
  Graphics
   SyncMaster (1280x1024@75Hz)
   4095MB NVIDIA GeForce GT 630 (ZOTAC International) 53 °C
  Storage
   931GB Seagate ST1000DM ST1000DM003-1CH1 SCSI Disk Device (SATA) 36 °C
   465GB Western Digital WD My Passport 070A USB Device (USB (SATA)) 42 °C
  Optical Drives
   ASUS DRW-24D3ST SCSI CdRom Device
   ROXIO DVD-ROM EMULATOR SCSI CdRom Device
   WD Virtual CD 070A USB Device
  Audio
   Realtek High Definition Audio
Operating System
 Windows 7 Ultimate 64-bit SP1
 Computer type: Desktop

If you could provide us with the following that would be great:

CPU: 

Motherboard: 

RAM: 

Hard Drive: 

Operating System: 

In as much detail as you can manage. 

Generally operating systems slow down with time as they become bloated with redundant software, mallware, viruses and other clutter that builds up over time. I assume the PC was much faster at some point, in which case it can be made as fast again but without knowing what operating system you are running no one can help you