RSSI or received signal strength indication can go from 0 to -100 (for Apple - other vendors can measure it differently as RSSI has no stated units. See Wikipedia)
Generally the higher (closer to 0) the better, and the closer to -100 the worse. Currently my iMac is 20 feet (line of sight) from my wireless base station and fluctuates between -57 and -58 and is connected at full speed. My laptop which I can roam around the house with never goes lower than -60 and never higher than -51.
The number to pay much more attention to is the Transmit Rate - the transmit rate will drop if you have too much noise or interference or if you are too far away from the base station. For reference:
0 - No connection
6 - Half 802.11b
11 - 802.11b
54 - 802.11g
130 - 802.11n on 2.4 Ghz
300 - 802.11n on 5.0 Ghz (Normally I see this connect at 270 for some reason)
Note: RSSI is a percentage and not a linear representation of how many dBm is actually reaching the card.
the 270 vs. 300: it is probably because of GI (Guard Interval), using a shorter guard interval will give higher troughput, but it's usually turned off -> probably because of compatibility. If you want to check it to be certain, find the MCS-index and look it up:
802.11n datarates are based on a number of factors (MIMO,GI,20 vs 40 MHz etc.), hence the MCS-index.