3 Gig works fine in pairs. Aka 2 X 1 Gig and 2X 512 meg.
However they must be the same speed and same vendor or they may have issues.
The 620 has 4 memory slots.
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NOTE: The ultra small form factor computer has only two slots which are considered an interleaved pair. The mini tower, desktop, and small form factor computers have four slots.
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A
matched pair of memory modules in connectors DIMM1 and DIMM2 (white securing clips)
B
matched pair of memory modules in connectors DIMM3 and DIMM4 (black securing clips).
