You can put 2x 1024 sticks in the spare slots mic, no probs. BUT.. It may lower the speed to 333 or even 266 with all slots filled, this is motherboard dependent. I'd just take out the 2x 512s and replace them with 2x 1024s. .2Gb is still more than enough RAM in my opinion... and they would still stay at their rated speed.
That's what I was thinking after I posted actually......
As for Motherboard specs etc, I'm pretty useless

ummmm.....I'm pretty sure it's only DDR.....tell me where to find out and I'll be able to give you a certain answer

Various specs lifted from CPU-Z
Chipset
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Northbridge NVIDIA nForce3 250 rev. A1
Southbridge NVIDIA nForce3 MCP rev. A2
Memory Type DDR
Memory Size 1024 MBytes
Channels Dual
Memory Frequency 201.0 MHz (CPU/12)
CAS# 3.0
RAS# to CAS# 3
RAS# Precharge 3
Cycle Time (tRAS) 8
Bank Cycle Time (tRC) 11
DRAM Idle Timer 16
Command Rate 2T
Memory SPD
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DIMM #1+2
General
Memory type DDR
Manufacturer (ID) Nanya Technology (7F7F7F0B00000000)
Size 512 MBytes
Max bandwidth PC3200 (200 MHz)
Part number NT512D64S8HC0G-5T
Manufacturing date Week 36/Year 04
Attributes
Number of banks 2
Data width 64 bits
Correction None
Registered no
Buffered no
Nominal Voltage 2.50 Volts
EPP no
XMP no
Timings table
Frequency (MHz) 166 200
CAS# 2.5 3.0
RAS# to CAS# delay 3 3
RAS# Precharge 3 3
TRAS 7 8
.mic