I love these older Dells for their cheap simplicity.
So you can upgrade to 8GB using 4x2gb units as long as you are on the most recent bios. You will probably be limited to ddr2-800 aka pc2-6400u. (This system does not use fbdimms so that is of no concern.)
Yep, and you can upgrade to the qx6800, which will pack a nice punch depending on what you are upgrading from.
As far as gpu, this is where it gets a bit tricky. While your power supply is good and can even be upgraded with almost any standard atx unit, the amount of room for the card itself is limited due to the btx motherboard design. This will be your biggest challenge. The other issue will be that newer cards will require a uefi bios, which the 9200 does not have.
Still even with these restrictions, there are a number of upgrades that are available as a single slot, short card:
Top Single Slot Graphics Cards from Nvidia and AMD for Gaming and Graphics Work. These Single Slot Video Cards are slim and occupy only a single slot on your motherboard or PC case. These single slot GPUs come with a slim heatsink and fan combination to keep temperatures down to safe levels...
graphicscardhub.com
Your 7900 gtx is actually pretty quick for what it is, and at best you will see a 2x-5x increase in gpu specs with an upgrade, but the actual performance may be limited to much less than that due to the cpu architecture. For now, I would hold off on a graphics upgrade until you get the memory and cpu in place.
And one thing to watch for regarding the cpu--watch out for fakes. It is better to buy a known working used processor from a reputable electronics recycler than a 'new' one from overseas. The 'new' ones are not only fakes, but could have all sorts of malicious intent packed in them.
I hope this helps and let us know how it all goes!