The 4200+ essentially has two Athlon 64 3500+ CPUs in it. When I say essentially, I mean that there are two cores in the die with a 2.2 GHz clock rate and 512KB L2 cache, like a 3500+. Unlike the Pentium Ds, which actually *did* simply have two Prescott or Cedar Mill P4 chips stuck under the heat spreader, the X2 has them all on one single monolithic piece of silicon. Look for a picture where somebody popped the IHS off of a Pentium D and an X2 and you'll see what I mean.
So since the 4200+ is two 3500+'s, 32-bit application performance is supposed to be similar overall to a 3.5 GHz Pentium D, which is pretty much is. The performance of the 4200+ is pretty similar to the performance of the Pentium D 950 (3.4 GHz) overall. As a result, in single-threaded 32-bit apps, your Pentium 4 3.4 GHz will perform rather similarly to the 4200+.
I happen to have a 4200+ and it runs very nicely under multithreaded loads, so your system configuration must be goofed up. Here's what I'd do (I assume you're running Windows, if you're running Linux, I can help you even more- just reply and say so.)
1. Open up the device manager and see if it says "ACPI Multiprocessor PC" in it. If it does not, then only one core will be used and that will cause the 4200+ to not really be much faster than the P4.
2. Keep an eye on the RAM usage in the task manager. If you are running short of RAM while doing a lot of stuff, the system will get slower and slower as it swaps until it just locks when all RAM + swap is gone.
3. Check to see if there is a virus or a memory-leaking hanging process/driver by keeping an eye on the task manager. I've seen this with Windows before, and the app will just eat up CPU cycles and RAM until it kills the system.
4. Keep an eye on the CPU core temp. If it gets too hot, it will throttle and eventually shut off when it gets too hot. Ditto for the chipset- you'll need something like Motherboard Monitor 5 to see that temp. Hot chipsets can act flaky too.
5. Did you install AMD's Dual Core Optimizer and have Cool 'n Quiet set correctly?
6. Try to look for a BIOS update as a really flaky BIOS can do weird things to a system.
Between the suggestions maddog and I said, you should be able to fix your system. If it doesn't, then perhaps taking it to a real professional (a mom and pop kind of store as they generally are the most knowledgeable) would be in order.
And to maddog, DDR and DDR2 are *not* slot-compatible, so you can't physically put DDR in a DDR2 socket and vice-versa as they will not physically fit.