Upgrading help needed

[eureca]

i am planning to upgrade my pc .

the config is

p4 3.0 ghz ht processor (775 socket)
dg 101 ggc mother board ( very bad mother board )
256 ddr ram

Would this processor work fine with ddr 2 rams and new mother boards , i also need a mother board with serial port , cos i need to run billing software at my office which need serial port for printing in dos format ( my software programmer is not likely to make any change in near future )

if the processor is not compatible please suggest me a good combination

Budget
proc+ ram + mobo under 8 K Rupees


Please help me , i need to do shopping soon

Thanks