OK, because I don't really know computer site / places that deliver to Singapore, I'll give you some parts etc to look out for, and also general rules when putting it together.
Generally, If you buy an AMD Processor, you will save yourself a bit (Not masses) of money, and get yourself more bang for your buck (more for your money!).
Best thing to do is start with the MOBO and A CPU (Processor) and build everything around that. You will want to get yourself a MOBO (Motherboard) that is DDR3 Compatible. DDR3 is the newest type of RAM that is on the market - and isn't much more expensive than the older types of RAM (DDR2 etc) as they are stop being produced as much. DDr3 is also faster, and compatible with more modern technologies. For your needs I would suggest 4GB of DDR3 RAM would be fine. Perhaps 6GB maybe, but definately not necessary. Pretty much all new MOBO's can be upgraded to hold more RAM at a later date anyway (Unless you buy a pre-built system from a company)
After that get a CPU that will fit. At the moment, and with your budget, you should probably be looking at either a decent quad core AMD Processor (Look at the Phenom II x 4 range, or the Newer Phenom 6 core processor), or if you are going intel, the i3/i5/i7 range. The i5 probably best, i7 might be out of budget. Intel also has the new sandybridge processors, in particular the i5 2500k is decent, and probably in price (But double check compatibilty with your board)
Once you have decided on those 3 (RAM, CPU, and MOBO) get a case, so you know everything is going to fit, and pick up the extra's like a decent branded PSU, heatsink, fans, graphics cards, drives etc, and anything extra like peripherals. (Keyboard, monitor O.S etc etc)
And a side note - on the O.S. front (Operating system - E.G. Windows) make sure you get 64bit version, so you can utilise all your potential RAM wise.
Ill give you a while to digest all that!