Oruboris, I have been doing a lot of researching and purchasing as I have been upgrading my computer. You have good questions. I would say that you would get tons more responses if you put this in general discussion, but here goes.
The first and most important question is, How much can you spend? This will dictate future options. I learned a long time ago that it is better to save than it is to gratify now and wind up weeping later.
Second, do you know someone who can build you a computer rather than buying a whole set-up from the likes of Best Buy, Wal-Mart, Dell, and Gateway? If you do count yourself blessed. It is far less expensive for a GOOD working model and a whole lot wiser to buy by the piece. This not only my opinion, but many people's opinions. This way you can build the computer you want. I bought a brand new Asus V9280/TD AGP 8X 128 meg Video Card (Asus also make high quality motherboards) for $135.00 from NewEgg.com with free shipping. I got a Pentium 3 1.2 Ghz chip for less than $100.00 from Googlegear.com and it had 2nd Day Free Fed-Ex shipping. In both cases I paid no tax.
What I am getting at is if you do the research, take your time, and search the web you will be able to build the system you want. I don't have the highest end system but neither is it low end. I got good - very good - high quality components and my system can take anything thrown at it -- a least for now.
Stay away from shared memory! If you can, get a motherboard without built in sound and especially video. However, it is becoming very difficult to find the latest motherboards for both Intel and AMD processors that don't have built-in sound. However, this sound can be disabled and you can get a SoundBlaster sound card.
Go with 512 megs of RAM for XP. RAM does make a difference. Make sure it is quality RAM By the way, how is your system becoming unstable? What is it doing? Have you scanned the disk and defragged the harddrive? Have you done a virus scan? Below are tech sights I highly recommend for your education. You won't understand a lot but your knowledge will increase in time:
www.resellerratings.com - tremendous sight for customer ratings of online computer retailers. Has a list of the most reliable and trustworthy vendors online. Also has a search engine for every computer part you could imagine. Check out their community forums and read about everything concerning computers from making choices to correcting problems.
www.tomshardware.com - Great site for industry news, reviews, as well as a forum. Very up-to-date in content.
www.AnandTech.com - another good site in the same vein as Tom's Hardware. Also is very up-to-date in industry content.
Hope this helps.