My ping used to generally be less than 150. I had 2M connection and it seemed to suffice just fine.

Only few days ago my ping started exeeding 400 in almost every game. Well I was curious, so I ordered a 8M connection, but still the ping climbs up to almost 450. And only after the matches have been on for a while, the initial value is always less than 50.

I've encountered no problems associated with high ping (lagging) and none of my game mates have complained (although I asked). Also, the result on speedtest.net shows ping as low as 18.

So what could cause this? Why does the ping seem very descent at first, but climbs up to astronomic values in a minute? 4 times faster internet connection seemed to make no difference whatsoever.

I suspect the ping shown is higher than it actually is.

I had ping problems too but I did not find out what caused them, they just disapeared. Just wait, crysis2 is a real bich when it comes to technical issues.

Btw. I higher bandwidth must not give you a better ping...

I hope you do know that Bandwidth and Connection are two different things? they are like voltage and electricity if you know what i mean . Let me clear the 2 meanings "bandwidth" and "connection speed" for yeh.
Bandwidth Is the limit of how much bytes per second u can transfer. For example ISPs(internet service provider) maximum bandwidth is 1gbit. Your maximum bandwidth is as you say 8 mbits (as you know you can get as much as 1 mBytes per second which is quite fast to me )
Connection speed Is how fast packets you can send and recieve. Connection speed is also called "ping", why? because i really don't know and my guess is when your packet reaches the server(which means your sending packet (upload ) ) it's kind of making virtual sound ping as in hotels when you call the guy with that metal thingy, the incoming packet( which you recieve or its download) pings you back saying hey you called me?
Imagine internet like this, Water pipe is internets bandwidth and connection speed or PING is pressured water. you can buy small width of your water pipe but you can make high water pressure so water is fast and goes far but it won't be big in diameter. You get the idea

I do know the difference. The fact that the ping always initially shows less than 50 (before the match starts), but rises rapidly after the game starts made me suspect that the bandwith isn't sufficient to transfer the amount of data required by intensive gameplay, thus resulting in higher latency.
And the fact that pingtest and speedtest, too, show my ping is less than 20.

So I assume it's safe to say that the problem is actually not my connection.

E: And I actually save 10 euros per month for upgrading the speed

ping is depending on quality of Internet connection and on range between you and server. Of course if provider is awesome then range is excluded. But now no one provider has awesome quality. It's only on paper, not in real.
I have best pings to nearest Moscow servers. Only 25-40 ms. But they are mostly empty, or busy. And I have to play on FR GE and other servers where's my ping sometimes 75-80 and sometimes 85-115. But this ping is still ok for the game. 400-500 will be bad playable cause it will be sightable for eye.
Funny thing is that on GE servers in list showing 1000 ping and on them 85-115))) Joke of packets maybe.

I play on FR GE servers, too.

The 1000 ping in server list on most of servers means that server list cant get the servers ping... its not letting server list to ping those servers which will be fixed in next patch.
400-500 will be bad playable cause it will be sightable for eye.
That statement is not true. What if i tell you that you can notice the difference between 20 MS and 100 ms? You won't see it but you will feel it. The feel is kind of gelloy and i cant explain it. Its quite fast but you feel like its kind of slow at the same time. I used to play WoW with like 250-500ms and when i switched to far far better provider i have 40-50 ms... and my god that felt good. Now when i get 100ms im feeling all gelloy and kind of slow but fast at the same time so you can notice the difference its really really weird

400-500 will be bad playable cause it will be sightable for eye.
That statement is not true.
In case If you didn't knew that our eye can react to something from 0.1sec to 0.3 sec. And in case of that 0.1 and 0.3 is a 100 and 300 ms. All that comes through that range become sightly visible for eye reaction. And if there will be 400-500 ms ping our eye will have plenty of time to react to the object quickly, due to the fact that the time is more than in range of 0.1 sec to 0.3 sec, for 100-200 ms. In that case, even quick reaction is don't needed. Cause there will be some delay, or in computer world called lag. And if there will be player with 100-300 ping and the one with 400-500. The guy with 100-300 will react quicker. And will kill other guy all the time cause his ping is equal to eye reaction time. And other guy will be frustrating about game, cause he has some delay(lag) for 100-200 ms behind the diapason of eye quick reaction time.