what Pc hardware should i upgrade to make my program run fast?

i have foxpro program and running a simple accounting program which includes posting of collection and ageing of accounts.

but what am concern is, the more i posted a collection, the slower the program computes of amount it totals. its like the program re computes everytime i add an item..and i want to make it run faster.. what should i do? should increase my RAM or upgrade my processor?

my pc specs are:
AMD Sempron64 3000+
512mb ram
256mb video
250gb Hard disk (IDE connection) /
(does SATA hard disk help increase the computation?)

thanks