I decided to upgrade my computer a few days ago (the reason is tedious...). In particular, the processor, motherboard, RAM, and graphics card will be 'gone' and I need to buy them. If you know exactly what you use the computer for, it is easy to spot what components you really need.
Processor
I often use VirtualBox. So, for reasonable performance, the processor must support nested paging. I am inclined to buy an AMD processor. Don't need to have very high processing speed.
For AMD processors, nested paging became available starting with the K10 architecture [en.wikipedia.org]. I checked some prices lists, and found that the cheapest AMD processor with K10 is AMD Athlon II X2 240, which was priced at RM198. Ok, would get this.

Motherboard
The AMD Athlon II X2 needs AM3 socket. The cheapest motherboard on the price lists that supports AM3, is stable, and has good name or reviews is ASUS M4N78 SE, which was priced at RM220. Besides, it is good that the motherboard has an UltraDMA interface, as my optical drive and a harddisk still need it. Ok, should be a good choice.

Graphics Card
The ASUS M4N78 SE motherboard supports only graphics cards with PCIe interface. I don't play high-end 3D games, so I decided to get the cheapest card out there. There are three cheapest cards priced at RM125, and I finally chose XFX NVIDIA GeForce 8400 GS. Both the motherboard chosen and this card used the chipsets designed by the same company -- Nvidia. Besides, the card supports GPU programming via CUDA. Ok, would get this too.

RAM
The motherboard supports only DDR2 RAM. Decided to get 4GB as I often run several big applications at the same time. Finally chose two pieces of 2GB Kingston DDR2 RAM, each priced at RM133.

So I spent RM809 to buy all these yesterday, and upgraded my box last night. The performance and features are satisfactory. I get a very significant performance boost too when using VirtualBox due to the support of nested paging. I'm using it to write this post with the enhanced box right now.