Well if the media is video, then a higher memory bandwidth is certainly going to provide a noticeable performance increase!
Also 256KB (not Kbit btw) isn't going to hold up against 2048Kb.
Both those reasons are why the Celeron was a cheaper processor, and if I remember correctly, they were just P4's with bad cache. I've got the same P4 chip at home just sitting there, I wont get enough money to make selling it worthwhile, so I imagine your decision (with regards to cost) should be very simple.
I don't have any benchmarks for the Cel 2.8 but I can say the Core 2 Duo E8500 @ 3.16Ghz is approximately 4x as quick as the P4 3.0 CPU (I used the same hardware and swapped chips). So if you're wondering about getting something a little newer without going crazy, the Core2 chips are a massive step up (and the new motherboards backward support the older P4 chips too, which makes upgrading to Core2 a wee bit easier).