Upgrading GT7600

The Radeon HD 5670 or HD 6670 would probably be the most I would upgrade to. The Athlon 64 3800+ is a single core CPU and will most likely bottleneck a faster card.

Additionally, the motherboard is a bit old and assuming it has a PCI-e 1.0 slot, it will have no issues with a PCI-e 2.0 card. However, it some old motherboards with a PCI-e 1.0 slot can be incompatible with PCI-e 2.1 cards.

There is a possibility that you may have a motherboard with the now obsolete AGP slot. Therefore, you need to find out exactly which version of the GeForce 7600 GT you have because PCI-e and AGP are not compatible with each other.

Assuming you have a PCI-e 1.0 slot, then the safe bet would be to buy a PCI-e 2.0 Radeon HD 5670 for $63 after rebate.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

If you want to take the risk that a PCI-e 2.1 card may not be compatible with your motherboard, then you can buy a Radeon HD 6670 for $65 after rebate. It is about 10% faster than the HD 5670.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

If you have an AGP slot on the motherboards, then the best card you can buy is the Radeon HD 4670 for $99 after rebate. It is approximately 25% slower than the Radeon HD 5670.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

If $99 is too much money, then you can buy the slower Radeon HD 4650 for $80. It is about 25% - 30% slower than the Radeon HD 4670.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Thanks very much,

You are right - the board has got a PCI-E 16 v1.0

Any thoughts on the benefit of more RAM with the above VGA cards?

I would go for 5670 or HD 6670 as well.
+1 to jaguarskx

You should upgrade to at least 2GB of RAM. Upgrading to 4GB would be better since some games can use as much as 1.6GB of RAM after a while because of memory leaks. Which leaves very little for the operating system. However, if you have a 32-bit operation system like Win XP or 32-bit versions of Win Vista or Win 7, then the operating system will only recognize around 3.25GB - 3.5GB of RAM.

Thanks guys,

I think that's what we'll do:
get a PCI e 2.0 HD 5670 plus upgrade to 4GB - everything else will call for a newer board