Well there are no new motherboards with a AGP slot, so thats problem no 1.
Also I guess you still use DDR instead of DDR2, no new motherboards with DDR slots anymore.. Your HDD is probaly a PATA one, if lucky it's SATA, but you can still find motherboards with 2 IDE connectors!
I recommend upgrading to a cheap am2 / am2+ motherboard with a AMD X2 serie processor, around 5000+ / 6400+ would be fine. Also the HD4770 is very cheap and has a decent preformance, sometimes even equal to HD4850. You probaly have to buy DDR2 memory to, but it's very very cheap nowadays.
Total costs will be around 200 / 150 euro's I guess.