Pingtest seems to give a fairly good idea as to how your 3G connection will handle FPS games. Speedtest seems to always display a lower ping in it's results, I've got Speedtest readings showing pings from as low as 33ms which is nonsense I've never EVER had a ping as low as that. My ping ingame most of the time correspond to the Pingtest results (The above is to a CT Server). As Graal mentioned it's more the stabilty (This is the Jitter in the above result) of the connection now days that is a issue.With the current 3G connections the latency is fairly low now days, still not any where near ADSL latency but it has improved.
I've had 8ta for a few months now and have played a number of FPS games online since I've signed up. The decrease in latency from my previous 3G connections was a big shock (A good one mind you). Vodacom and Cell C gave me 100-130 ping ingame, Vodacom had High Jitter, Cell C had Low Jitter.
Vodacom = RSSI of +93dBm
Cell C = RSSI of -63dBm
8ta = RSSI of -77dBm
My 8ta Jitter is between Cell C and Vodacom, which in my opinion seems to be related to the RSSI of the three service providers.
So a lower RSSI should give a lower Jitter and hence a more stable ping <-----
This is only MY opinion 
1. I'm from Port Elizabeth
2. Between 6-7Mb often goes higher
3. This is a difficult one probably lowest around 150KB highest 900KB I'm not a big downloader
4. 45ms-50ms to CT Servers / 65ms-75ms to JHB Servers (I don't feel any Jitter on my side, so if there is, it's very small)
games.saix.net
Pinging games.saix.net [196.43.22.222] with 32 bytes of data:
Reply from 196.43.22.222: bytes=32 time=68ms TTL=24
Reply from 196.43.22.222: bytes=32 time=67ms TTL=24
Reply from 196.43.22.222: bytes=32 time=65ms TTL=24
Reply from 196.43.22.222: bytes=32 time=74ms TTL=24
Ping statistics for 196.43.22.222:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 65ms, Maximum = 74ms, Average = 68ms
server24.eu
Pinging server24.eu [217.70.146.57] with 32 bytes of data:
Reply from 217.70.146.57: bytes=32 time=216ms TTL=51
Reply from 217.70.146.57: bytes=32 time=215ms TTL=51
Reply from 217.70.146.57: bytes=32 time=214ms TTL=51
Reply from 217.70.146.57: bytes=32 time=213ms TTL=51
Ping statistics for 217.70.146.57:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 213ms, Maximum = 216ms, Average = 214ms
The above two results are some what misleading I do not have a rock solid ping like they portray in the result. Like you mentioned, testing it in game on a few maps will probably give you a better idea as to how
your connection will handle the FPS games online.
Good Luck!
