I'm currently having a load of problems mapping. I think this is basically my computer isn't good enough...

The problem I'm getting is the system freezing while mapping. This tends to happen when I use the scroll wheel to zoom out of a map wireframe view (x/y, etc;). And it's happening more the more complex the map gets.

Currently run:
1700+ Athlon
768 MB DDR Ram
GeForce2 GTS

As you can see I need to upgrade, but is there anything I should prioritise?

imho, i'd say:
1) RAM (and lots of it.. like... 1gb or more)
2) Vidcard (get a better one.. esp for dods)
3) cpu (faster is always nicer)

Right now, I'd say your RAM is the best thing you have in your rig....

In terms of upgrade, let's put it plain and simple, you'll need a new vid card to play Dod Source.

As far as CPU is concerned, a 2 GHz is the minimum requirement for Source, so there you go, you can prepare the credit card too...

If you don't have enough money for more ram, keep the one you have (unless it's not running at the proper frequency for your new processor, which is very likely)

you can improve performance a lot without an upgrade. for a start, use the visgroups. make your own groups and filter out anything you don't need. if you look at the SDK maps, they have groups for the 3dskybox, certain sections of the map etc. or you can use the cordon tool to select one part of the map and hide everything else, which is especially useful if you're placing props, but can't even see what you're doing in the 2d views with thousands of other lines all over the place.

something else i've noticed is that mapping for CS:S is a lot laggier than mapping for HL2, which i assume is because CS:S uses it's own gcf, aswell as the HL2/HL2DM ones, so before DoD:S is here, you might be better using HL2. use filters in the texture menu aswell, scrolling through to find a texture and loading hundreds before you get to the one you want stores them all in your memory i think, and after a while my pc slows down as much as when i'm compiling.