Well, first, do you know for sure what parts do and do not still work? Replacing the CPU for sure, based on the previous issue, is highly recommended. I would say the following parts:
Asus A8N-E ATM Mobo $110.00
AMD Athlon 64 3200+ $160.00
For a video card, I recommend an ATI FireGL card for the type of work you do. For gaming, I'd never in a million years recommend ATI, but here it's much more fitting. Has better OpenGL support for CAD and 3D work. You can pick up a 128MB card for about $175 or so... PCI Express.
Your memory may transfer over. If not I recommend
Mushkin SP3200 512MB chips for $40.00 each. You don't need more than CAS3 for that type of work (again, a different type of recommendation from gaming). The 3-3-3-8 timing will be good, and you should see a memory improvement over PC2700. Also this is $40 for one stick, meaning it's an easy, and relatively cheap upgrade to 1GB of RAM. Crucial, Corsair, Geil and OCZ are also reputable, solid brands of memory. I've used Muchkin for years, though, and prefer them overall.
As for harddrives, Maxtor, WD and IBM/Hitachi are decent... for SATA drives, I tend to recommend Seagate. If you're mirroring drives, you won't increase their size, you'll just have a 1 for 1 backup. Maybe you already understand that, just clarifying. Failure rates are roughly the same as they've always been. Seagate is the most reliable, currently, but also tends to be a touch more expensive. If you back up your data some other way, get a larger drive. But then remember that it's that much more to back up. Getting 2 decent size drives and mirroring them is always a good option. The ASUS board, of course, has RAID 0, 1 an 0+1 on board.
As for power supplies, I generally recommend Antec, though ThermalTake and Sparkle are both great brands as well. The
Antec SmartPower 350W goes for about $50 and is decent. If you have a few more bucks to throw at it, go for a
Antec TRUEPOWER2 480W for $85.00. If you really want a high end power supply, try the
Antec Phantom 500W Silent for $160.00.
As you can probably tell from the links, I also recommend NewEgg.com for buying online. VERY fast and inexpensive shipping, good customer support, easy to RMA through if something comes DoA. I think many ppl here would recommend them as well.
At any rate, if you have any other questions, feel free to ask here or PM me. I can get any data you may need to back up product claims and such. Good luck!