Heya Jorm,
Well, if you have a high-speed over a cable-modem, then the key thing is this. Your 'maximum' theoreticaly download speed of 6.0 Mbps, does not take into account packet overhead (up to 10%) and what other people on your block are downloading at the time.
Yup, cable-modem's share the total pipe with everyone on that leg to the cable box in the street. If your neighbor down the street is online and streaming HD porn feeds to burn onto his BlueRay DVD set, then you are not going to see your 'maximum' speed.
There could also be some test specific issues like a slow test server, or your media player in the background sucking up cpu cycles. Try this. Turn off all applications, all music players, streaming audio feeds, and make sure your Anti-Virus is not currently scanning. Then, go here and try several of the free speed tests.
http://www.dslreports.com/stest
You should see some similar speeds, and also note what time of day you do the tests. At this time of night, I'm lucky to see 668 kbps from my 1.5Mbps connection.. although in the morning, I have gotten 2.3 Mbps ... well above rated... (which is the positive side to sharing the neighborhood).
Also remember, that 'kbps' means kiloBITS per second, not bytes. So you need to divide by 8 in order to see how many bytes per second you are transferring. (i.e. 668kbps = 83.5 KB/sec).
One final thing to throw into this brain dump of mine.... MMORPG are engineered to use less than 2.5 KB/sec to make all that magic happen. (I believe UO was as high as 2500 bytes/sec, and EQ1 more like 1400 bytes/sec). That means, that while your overall download speed may be less than your 'maximum theoretical', you still have ample bandwidth to play WOW, EQ2, EQ1, DAOC, UO, etc.
Now packet loss... well that is really bad, and something else alltogether. Dig around the internet and find the name of the server you play on (i.e. Thunderlord, etc.) and open a DOS PROMPT window and type 'tracert thunderlord.blizzard.com' or whatever.. It should show you each computer 'hop' between your computer, through your ISP, and onto the internet and finally to your server. If see any '*' on a particular computer connection, then that is where your packet loss is coming in.

Apologies if I misunderstood the question... and just pointed out some stuff you already knew...

Hope that helps!
~Urgon
p.s. DSL lines do not share the bandwidth, and will have more consistent speeds around the theoretical maximum .. but cost more for the same speeds... (typically)...