Look at the 3rd digit from the end.
(e.g. the 2 in 4200, the 3 in 4300, the 4 in 5400, the 7 in 5750, the 6 in 3650)
That number gives you an idea of the power of the card. The least you'd want for a gaming computer would be a 5, and a 6 or more would be better.
The first number tells you the series.
(e.g. the 5 in 5400 shows it's a newer generation card than the 4200)
Current generation for ATI is the HD 5000 series. So a 4200 is still fairly recent, though it's a low end card and may be lacking features. You might find the 4200 is a little slow with some games (leading to "sticky cursor" or other rendering problems related to speed), especially the laptop version.