to elaborate a little more (and also agree with JDW81), IBU's are what you need to know about because one hop may have an Alpha Acid content (AA% - very simply put, the bittering potential for a hop) of say 3% while another may have 13%. If you made an IPA with a kilo of the first hop it would be VASTLY different from using a kilo of the second hop (which would produce a beer roughly 4 times more bitter than the first).
Even within the same variety, or from the same grower over different seasons there is natural variation in the AA% of hops. So to think of it in grams per volume is only useful if you know the AA% and can determine IBU's.
Al
Edit: Before I start a war or get flamed to death - Obviously also the wort gravity, addition times, type of hop product and a bunch of other things effect the way your hops work and contribute to your beer. But start with the above and expand from there.