Answering your VISTA apps question-
1) You will find much of your existing software is incompatible with VISTA unless you bought an upgrade in the last 9 months or it is freeware. (Perfect Disk, Nero, Office 2000, are just a few from my bookshelf that will not work, ever. That is $650 in software I need to replace if I go VISTA.)
2) Warrantee... Not worth the paper they are written on unless they offer in home service. If you mail a part out it will be 3-12 weeks before your PC is working again. Not acceptable. Last I heard Dell was still offering in home but I think it is extra $$$ to extend it to 3 years.
3) Get the DAMN 8800 GT! It is the only great choice out there and besides, we all want to hear from someone who actually has one in hand and is running it.
4) Do not assume you will be able to dual boot VISTA and XP on an OEM machine if you don't like VISTA or it is keeping you from running something critical. The OEMs are not offering the latest hardware drivers for VISTA and XP on VISTA machines. You may end up with limitations. If you build your own you can be sure of VISTA and XP compatibility.
5) HD's, 6, ... Most new machines (especially OEM) will only support 2 IDE and 4 or more SATA so if you have Optical and multiple IDE HDs then you will have to build your own or add an IDE expansion card (2-4 additional IDE slots for $30). Watch the MB's closely for number of IDE connections!!!
6) Pricing - Dell will offer rock bottom pricing on low end machines but expect to pay extra for a performance machine. That is how they make there money - give away the Barbie dolls and charge a fortune for the dresses. For a decent gaming machine price will be the same or lower if you reuse your XP liscence, case, mouse, kybd, etc and build your own. You will get a decent monitor bundled in with the Dell system.
It's a tough call but I would build my own knowing what I know now, even if it costs a little extra. Control of the subcomponents is crucial to having a long life computer, which with 6 years on your old machine, is what you're into.
7) CPU/MB choice - AMD is limping along and making too many socket changes for my taste, but they are offering about $100 cheaper for similar performance between the MB and CPU prices. The good news is a mid-range CPU/MB will run everything great.
8) Memory -
DDR2 is dirt cheap and dropping. I've seen 2 Gig of good memory for $50 after rebates.
9) Sound cards - why waste your money unless you are an audiophile.
Good Luck.
PS Why not just fix the old one? It's either the MB ($50) or the PSU ($50).