hi mate
i have similar specifications to yours,socket 478 epox 4plmi motherboard,geforce 7600gt agp,2gb ddr pc3200 ram,and a 3.2ghz pentium 4 (ht)
however the biggest problem you are going to encounter is finding a socket 478 motherboard that supports pci-express,it wouldnt make any sense getting another agp based motherboard
however theres some manufactueres(god bless em) that still make pci-express based socket478 motherboards
a excellent example is the Asrock P4VM890 PCI Express x16 socket 478(http://www.pcupgrade.co.uk/productdetails.asp?ProductID=3702&categoryid=259) and the ASROCK SKT-478 P4VM900 S/V/L(http://www.pcb-plus.co.uk/store/product/1593/ASROCK-SKT-478-P4VM900--S-V-L/),and to be honest these are about your only choice
but these motherboards are excellent and will bring you into the world of mega fast graphics with the pci-express x16 interface
however when changing the socket 478 processor make sure you are careful when removing the processor,the socket 478 fan and heatsink assembelys are difficult to disamantle,and since the actuation lever is hidden by the massive heatsink this can become a bit difficult
however heres a tip: when you have uncliped the fan assemebely take hold of the heatsink and gently move it ,only ever so slightly until you hear a "pop" or "crack"..this will be the "thermal compound" releasing its hold on the socket 478 pentium 4
you are now safe to remove the fan assembely and heatsink,then raise the actuation lever and gently remove the socket 478 pentium 4
when you fit the processor on your new motherboard use a more efficent and easy to remove heatsink/fan such as the Zalman CNPS7500-ALCU CPU Cooler (Socket 478/754/939/940/AM2/LGA775)(http://www.overclockers.co.uk/showproduct.php?prodid=HS-030-ZA&groupid=701&catid=57&subcat=821&name=Zalman%20CNPS7500-ALCU%20CPU%20Cooler%20(Socket%20478/754/939/940/AM2/LGA775)
this is a very efficent fan and the heatsink is in a bowl like shape alowing for better circulation,the fan also has a Fan Mate 2 controller allowing you to change the speeds..a excellent fan
also consider the Zalman CNPS9500A-LED CPU Cooler (Socket 478/754/939/940/LGA775) from http://www.overclockers.co.uk/showproduct.php?prodid=HS-015-ZA&groupid=701&catid=57&subcat=821&name=Zalman%20CNPS9500A-LED%20CPU%20Cooler%20(Socket%20478/754/939/940/LGA775)..nice design and excellent cooling potential,this is popular with overclockers and pc modders,they are also easier to replace and clean
as for a new case,well it depends on the size of the new motherboard,the asrock boards are both m-atx which should fit in any case,thr atx size sometimes requires a larger case
a excellent case upgrade is the Apevia X-Plorer Black/Blue Neon Midi Tower (http://www.overclockers.co.uk/showproduct.php?prodid=CA-030-OK&groupid=701&catid=7&subcat=505&name=Apevia%20X-Plorer%20Black/Blue%20Neon%20Midi%20Tower%20No%20PSU),this is a excellent pc case with some nice touches,these include a rear and side panel fan,a temperture monitor and a cool looking front access door
i hope this helps,however any problems let me know
good luck mate !