Assuming you're getting another 2*1gb pair, put the new pair into slots 1&2 on riser A and move the 512Mb dimms to slots 3&4 on riser A.
The RAM in slots 1&2 on both risers will perform slightly better than the RAM in 3&4. The DIMMS in 1&2 effectively forward the requests to 3&4 delaying it slightly, so you want the most dense ram in slots 1&2