A Pentium 630 is not powerful enough to run current games and the PCI Express slot on that motherboard most likely is x4 only. You can't expect to get much for a system based on a $10 CPU and a motherboard that was probably built for an OEM. Build it for the experience, but don't expect to make money on it. Actually you'll lose money if you need to buy components to build it.
Unlike what I originally thought, it only has PCI slots making it useless for gaming. It can only be used for office applications. Build it, but with such poor specs, you can't expect to get much for it.
The parts that you have don't seem to work together. Did you verify that the Asus P5GL-TMX/S supports the P4 630 CPU? Unless you already bought it, I'd pass on this OEM motherboard for the lack of support.
The MSI 915GM Speedster is socket 478; therefore you can't install a P4 630 (socket 775) on it. Buying a socket 478 CPU just to use it doesn't make much sense, but you may be able to get one for free (or for $10 if the HSF is included).
The Intel Celeron 900 CPU is a bit better, but you need a motherboard that supports it and it still isn't fast enough for a gaming system. The first Core 2 Duo (E6300) is much faster and they can be bought for $20 or less.
I'd suggest you get rid of what you have and buy parts to build a Core 2 Duo.
Core 2 Duo E6300 1.86GHz - $20 or less with stock HSF
Socket 775 Motherboard that supports a Core 2 Duo - good luck finding a used one
- 2GB or 4GB of DDR2 memory - $30 or more
- Hard disk
- Cheap case
- 400W power supply - $40
- Decent gaming card - $80
- Keyboard, mouse, etc.
No matter what you decide, building a PC based on old technology is a bad investment. For a few dollars more (see http://www.anandtech.com/show/5005/holiday-budget-syste...) you could build a much faster system if you buy a new CPU, motherboard, memory (DDR3 is much cheaper than older DDR or DDR2) and PSU. Then buy used or inexepnsive components to complete your build.