You install ONE DDR2 RAM ???
Nearly all motherboard will need to install DDR2 in PAIRS, i.e. 2x

Check if your old DDR2 are in PAIR also, ie 2x

Normally it is NOT advisable to mix different brands of DDR2 together in the same pair

Mixing different brands of RAM is pure luck
First their latency must be exectly the same, ie CL5-4-4-12
Secondly their chipset should be the same, ie 4x or 8x units, since this impact the data access

I would recommend you to use same brand and model for the pair.

BTW, Windows XP or Vista normally can identified just under 4G (ie 3.XG) of RAM as a 32-bit operating system.

As far as I understand you have 4 options:

1.  1-pair (exact model) of 512M, plus 1-pair (exact model) of 1G RAM.  This wil give you 3G of usable memory.
2.  1-pair exact model of 1G RAM, plus another 1-pair of another exact model of 1G RAM.  This will give you around 3.5G of usable memory (due to 32-bit OS limitation)
3.  2-pairs (i.e. 4x) of exact model 1G RAM. You will also have around 3.5G of usale memory.
4. 1-pair of exact model 2G RAM.  This will also give you 3.5G of usable memory.

Of course you can choose not to do anything to save money.  My recommendation is that if your original motherboard already has 2G of RAM, it will not be worth to add anymore since you won't find any major improvement.  2G should be well enough for most application.

What is your motherboard ?
One of the common mistake is the order to lot-in those RAM chips.
You must have the motherboard manual to be absolutely certain.
e.g. normally if the 512M is type-A and 1G is type-B, then it is a common design that the chips will need to be arranged as ABAB or BABA.  Do check the manual.

Are you sure ?
This motherboard only allows 2x RAM chips, not 4x

Your motherboard only allows ONE-pair of RAM
ONE-pair means 2 slots, not one

If you have one 1G and one 512M, you get 1.4xG is normal since the system takes away a little memory space for internal use.

So I suggest you to buy one more 1G RAM, together with the existing 1G to make a 2G computer which should be sufficient for most Windows applications.  Remember taking the existing 1G RAM to the shop and ask the shop to sell you a MATCHING 1G RAM.

The other option is to buy 2 new 2G RAM, but a total of 3.XG won't give you a lot of performance gain unless you use applications like Photoshop or AutoCAD.  So save your money.

You are still using Celeron.
You will see the different if you upgrade it to P4 even with similar speed.

Good luck.