Well, I think you'll get a considerable performance bump from it, since it's both a 75% speed increase and a move to a 133MHz bus speed, but whether it's an upgrade that can be justified on a cost basis is a different story. T-bird 1.4GHz is going for 73 dollars on Pricewatch, and a 1.33 is 44 dollars. An Athlon XP 1700+ (1.47GHz) is as low as 46 dollars. For the same price as the 1.4GHz, you can get an XP 2100+. Of course, that isn't any use to you right now.
You might consider getting an XP1700+ and a new motherboard, like the K7S5A Pro which will let you keep your current memory. That board will be able to last you a good bit longer with a better upgrade path for the CPU and later you can use DDR memory. Or you may want to make the move up to DDR right now and get the Asus A7N266-VM. It's certainly a better board, costs about the same, but requires you buy new memory.
Your video card isn't nearly as much of a need to upgrade as the CPU right now, so don't even consider that until you decide what you want to do about the CPU. The memory also isn't TERRIBLY important; though moving to DDR can provide you with a good performance increase, the CPU is still the most important.