The docs for the system should have specified the type. Cheapest memory is rarely the best memory. I prefer Cruicial, Corsair or Kingston - none of which are particularly cheap. I have reports that OCZ, Shikatronicsand Viking are good as well though I have not tried them. For pricing I'd try newegg.com first as they seem to be very competitive. Or visit pricewatch.com and see what you can find there.
I'm guessing that this machine takes unbuffered nonECC type memory running at DDR333 / PC2700 speeds but you should consult your manual for sure. If this is correct, I would suggest purchasing DDR400 / PC3200 since they are about the same price nowadays and would run at slower speeds too. I would also buy from manufacturers that offer lifetime warranty (like Corsair and Kingston).