I would just buy another 2 512's at 400. If you can't afford it try selling the 333's. The setup with all four is pointless, after all why pay for 400mhz when its not gonna run there? Though depending on how much memory you actually need ....
If you need the capacity, use all four (or get another 2 400's)
If you need speed just use the 1GB (or get another 2 400's)
In conclusion ... get another 2 400's
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If you need the capacity, use all four (or get another 2 400's)
If you need speed just use the 1GB (or get another 2 400's)
In conclusion ... get another 2 400's