250 is better than a 230.
To explain, the first number is model, like 2xx, or 4xx, or 9xxx.
The other numbers dictate performance level (in a general manner of speaking).
A GTX 285 is better than a GTS 250
A GTX 480 is better than a GTX 470
When looking between series, that's where it can get confusing.
EX: Is a GTX 460 better than a GTX 285?
That's when you want to look at stuff like benchmarks, total power draw, etc. Reviews can be looked at on many different sites, one being tomshardware. Other sites such as Guru3d.com are pretty good reference tools as well.