Yes. The E6420 is DUAL core, so you get two CPU cores and as each core is faster (much faster in video encoding) than the P4 630, you will get much better performance in video encoding, and somewhat better performance in gaming. And dual core will make your system feel more responsive under heavy load.
Overclocking the E6420 (which is easy) will make the difference even bigger.
2.5 will be nothing you can probably do it at all stock settings to. I'd shoot for 3 or 3.2 which with teh Conroe core it should be east or relativly easy to hit 3.2. Just get a nice cooler. A large heatsince will help dispurse the heat even in warmer weather and make sure to have fans to keep the air circulating. I dont have AC and it was up to about 85F the other day and my idle temps went up about 3C. Ambient temps do play a roll but if you have good cooling it should be a minimal concern.