What are your full specs OP? CPU, PSU etc
Also the external hard drive you keep mentioning will limit your downloads if you are downloading direct to that. There are a few factors when it comes to downloading which would be. Your isp, down/upload speed, where you are actually downloading from and the overall bandwidth of your system. For instance if you downloaded to internal SATA 2 HDD it would recieve data better than an external SATA 2 HDD due to it usually having to go over a USB connection. Plus you mentioned you have a 500GB SATA HDD. Is that just standard SATA and not SATA 2/3? If so then there is another factor to take in to consideration.
As also mentioned 8GB of ram these days is normal for gaming not because games will use it (Most don't get near 4GB. Very few infact) but will allow your systems OS to cache more from your HDD to your system ram. Therefore giving your programs/games faster access overall for your system.
Another thing already mentioned is your GPU is a low performer. One, because it only has GDDR3 (And not GDDR5 like the more powerful cards). And two, because you are running dual screens off such a low end card. When you say your card is "AMD Radeon HD 7340 (4 GB GDDR3, up to 2 GB)". You mean it's a 2GB card but can use up to 4GB. Which basically means if any application/game uses over 2GB of your graphic cards dedicated memory it will then utilise what it needs from your systems memory. Not really such a good thing considering you are only running 4GB total system memory.