I'm also doing my first build, my limit is 800 bucks as well and this is what I have gathered so far, Its two different setups, there not very different but here they are.
Setup 1 Price:$787,nothing but tower
Intel Core 2 Duo E6300(2x1.86ghz)
ECS 965T-A P965 Chipset(Quad-core)
512MB DDR2-800 PC6400
Nvidia GeForce 7300GS 256MB
Setup 2 Price:$772, just tower
Processor:Intel Pentium D 950(3.4GHZ)
Same As Above
Nvidia GeForce 7300GT 512MB
Thats what I have gotten from the very little reading ive done yet. Maybe it can help you and give you some sort of idea.
Also can anyone who has expierence, let me know if there is something I should change out. Is the mother board I have chosen ok for this setup?