The chip is good, but a 2.6c might be an option too: only slightly more expensive. The mobo is good too, and has a nice auto-overclocking feature when the processor is very busy; you might wanne think about a Asus p4p800 too, it is practically the same price, and comparable in features - it is just easier to overclock.
The memory: you could try cheaper non-twinX memory. It is rather pricy for such a system. Try normal cas2 low-latency memory instead - that should work in dual-channel mode without any problems, and would be far cheaper. I know - we sell those p4c motherboards in our computer shop, and have installed "normal" and TwinX memory in them without a problem.
(Oh, and do NOT go for the lowest-cost case possible. They typically have the worst power supplies. You need enough power to feed your system. If you do not have enough, your system will crash randomly, which would defeat the whole purpose of upgrading in the first place...)