Memory Upgrade!!

I just use my computer for internet browsing and running specialist applications related to outdoor pursuits, such as mapping and recording GPS trails.
Over time, i have noticed that the computer isn’t keeping up with modern uses for the internet, such as Youtube and Google Earth. I have upgraded my internet connection to broadband from a quality provider, however the computer is still noticeably slow when performing a number of tasks.
For the most part, I hope to do much of what I use the computer for now - just a lot more quickly. My digital video camcorder has a Firewire output port, and I want the new computer to be able to take video from the camcorder and create DVDs from it. Last year, a new hard disk was installed; and some time in the near future i plan to buy a new, larger, flat-panel monitor.
My current computer system has the following components:
Case – AOpen HX-45, Midi-Tower ATX Case
Power Supply – AOpen ATX Power Supply, 350 Watts, ATX12V Connector
Motherboard – Gigabyte GA-8IGX
CPU – Intel Pentium 4, 1.8 GHz
Memory – 384 MB (256 + 128MB)
Graphics Card – Integrated On-Board Graphics
Hard Disk – Seagate 160GB Parallel ATA
Optical Drive – Sony CD Writing/DVD-Reader

Special Attention should be made to ensure that the following facilities are available ont eh new system:
A serial port: for connecting the GPS unit
FireWire (IEEE 1394): for connecting the camcorder
Ethernet: for connecting the broadband modem
DVI Port: for connecting a flat-panel monitor in the future.
For software compatibility reasons, the upgraded system will continue to run Windows XP. (I already have a retail boxed copy of Windows XP Home from my current PC).
The proposal should be cost effective (i don’t want to spend money where it isn’t needed). I just want a series of upgrade decisions that will modernise the system, and provide the additional features necessary for my planned uses for the upgraded system.



Out of the following Memory which one would suit my needs and why:

DDR DIMM Modiles
1024MB DDR-400

DDR2 DIMM Modules
1024MB DDR2-800
2048MB DDR2-800
4G DDR2 1066

DDR3 DIMM Modules
2048MB DDR3-1333
4096MB DDR3-1333

DDR2 SODIMM Modules
1024MB DDR2 SODIMM
2048MB DDR2 SODIMM

Thank you very much for your help!