PCT Controller described a loss of separation event between BWI and DCA departure aircraft; acknowledging altitude separation would have prevented the incident.

Narrative

Air Carrier X was a BWI departure climbing out on the TERPZ2 SID. He was assigned FL230 and was climbing out of 11;500 FT as he turned south on the TERPZ2 at WONCE. Air Carrier Y was a DCA departure climbing out on RADAR vectors approximately 10 DME NW of DCA out of 5;600 FT. I issued Air Carrier Y a climb to 17;000 FT and a heading of 300; which he read back. Approximately 1-1/2 minutes later; I saw Air Carrier X was still only at 11;800 FT and Air Carrier Y was converging; climbing out of 10;600 FT. I assigned Air Carrier Y 12;000 FT. I then called traffic to Air Carrier X; who reported Air Carrier Y in sight and advised he was getting an RA and was climbing. I told Air Carrier X to maintain visual separation from Air Carrier Y and he said he would. Air Carrier Y then advised me he was responding to an RA with the aircraft off his right (Air Carrier X). I asked Air Carrier Y to repeat; he said he was in the middle of an RA. I told Air Carrier Y to climb to 17;000 FT when he was finished. I was aware of Air Carrier X's altitude when I climbed Air Carrier Y to 17;000 FT. I believed the 300 heading would keep Air Carrier Y separated from Air Carrier X laterally. I also felt the 6;000 FT difference in their altitudes when I issued Air Carrier Y a climb would assist in maintaining separation. While not a solid technique; I had no concern that the aircraft would close such a large vertical distance in such a short time. However; Air Carrier X leveled off at 11;500 FT and proceeded to only climb 300 FT over a 90 second time period just before the incident. By the time I recognized his low altitude; there was not enough time to prevent the loss of separation. On a human factors note; I believe that my knowledge and experience with the (old) TERPZ1 Departure out of BWI contributed to this incident as well. On the TERPZ1 HAFNR transition; aircraft did not fly over JIMME after WONCE; but instead turned more southerly towards HAFNR. On the TERPZ2; all aircraft fly over JIMME after WONCE; which is a more southwesterly track; before continuing on the various transitions. This failure to process the wider track of Air Carrier X (leading to greater time needed for Air Carrier Y to cross the projected path of Air Carrier X) gave me a false sense that Air Carrier Y would pass in front of Air Carrier X with lateral separation.Recommendation 1) Use vertical separation when climbing across the path of another aircraft and the possibility exists that there won't be 3 miles lateral separation. 2) Be vigilant of the new track of the TERPZ2 HAFNR transition. 3) Increase scan rate to more quickly recognize when aircraft fail to perform as expected; Air Carrier X climbed a total of 300 FT over a 90 second period just before the incident.