Ram memory

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I want to upgrade my son’s computer. His video card is an ATI Radon X850 XT AGR AGP 8X/4X. Currently he has 2 Corsair Ram Cards, CMX512-4400C25 1 GIG total. I have been informed from Corsair that this card is no longer available. His Mother Board is a K8 Triton Gigabyte GA-K8NSC-939 nForce3 250 Gb . It has two more empty slots for RAM Cards. The processor is an AMD Athlon(tm) 64 processor 4000+ 2.5 Ghz 1.00 GB of RAM. I built this machine about 4 years ago. I want to upgrade it to 2 gigs of RAM. Can anyone advice me where I can purchase another compatible RAM Card that would be the best solution to upgrade to 2 GIGS of RAM or do I have to take just completely change the RAM?. I heard different RAM types do not work well together. He uses his machine primary as a gaming unit. Any other suggestions would be appreciated to upgrading his computer but there is a cost factor as always. Need the most performance for the least amount of money. Sort of between jobs currently. Thanks for any help you can give me. Please try not to laugh.
New info: (maybe)
Here is more info on the Ram:
XMS XTRE ME MEMORY SPEED DIMMS
CMX512-4400C25 XMS4400 512MB 2.5-4-4-8 184 DIMM Black

• Mfr Part Number: CMX512-4400C25
• Type: DDR
• Capacity: 512MB
• Speed: PC4400 550MHz
• Pins: 184-pin
• ECC: No
Thank you
Aubrey Rogers