DDR and DDR2 modules have the same physical size, but DDR modules have 184 contacts, while DDR2 modules have 240. On Figure 1 you can compare the difference between DDR2 and DDR edge contacts.

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Figure 1: Differences on edge contact between DDR and DDR2 modules.

Thus there is no way to install a DDR2 module on a DDR socket and vice-versa.

All DDR2 chips use BGA (Ball Grid Array) packaging, while DDR chips almost always use TSOP (Thin Small-Outline Package) packaging. There are DDR chips with BGA packaging on the market (like the ones from Kingmax), but they are not so common. On Figure 2 you can see how a TSOP chip on a DDR module looks like while on Figure 3 you can see how a BGA chip on a DDR2 looks like.

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Figure 2: DDR chips almost always use TSOP packaging.

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Figure 3: DDR2 chips use BGA packaging.