I used to have this processor and very good it was too. Eventually it ran out of legs and I upgraded to Core 2 Duo 8400 when software started to require 2 cores.
I'd say really you need a new PC because to get everything up to date you'll have to upgrade everything. But if you're not too worried about being bang up to date, you can probably get a significant performance improvement for very little outlay.
Starting with the processor: processors that would fit your motherboard are no longer on sale, but you can probably pick up a dual core Athlon 64 4800+ x2 on ebay quite cheaply. Other options would be the 4600+ and 4400+. It must be a socket 939 processor.
A 200GB HDD can be had for a trivial outlay. Check what interface your motherboard has, either IDE or SATA and make sure your new HDD has the same interface.
RAM upgrade to 4GB is a good idea, but check your motherboard specs for exactly what RAM you need. I expect DDR2 SDRAM is what you need.
There's little reason not to upgrade to Windows 7 these days, but get the 64 bit version, otherwise you won't be able to make use of your 4GB.
I have a Blu Ray re-writer but hardly ever use the writing capabilities. I'd stick with your existing setup.
If you are into gaming you should probably also upgrade your graphics card.
The above would give you a significant boost but you'd still end up with a PC that's modest by today's standards. I'd say you should be looking at a new PC but obviously that would be much more expensive.