1 GB PC2700 or 512 MB PC3200?depends on what you're after and your needs. If you do a lot of memory-intensive tasks and OC your CPU is not a priority then take the PC2700. If you're more after performace and do intend to OC then get the DDR400.
However, if it was me, I would get the 512 DDR400 now and get a second stick at a later time.
However, if it was me, I would get the 512 DDR400 now and get a second stick at a later time.