Embora não seja uma resposta direta à sua consulta de uso de memória WOW64, sua 'RAM perdida' no Windows 7 pode ser devido ao armazenamento em cache com o qual você não está acostumado a usar o Windows XP. Por exemplo, " SuperFetch ".
Aqui está um bom artigo explicando porque parece que o Vista e o 7 usam muito mais memória do que o XP.
Blurb:
...it was little surprise that, upon checking my reported stats on XPnet, I found that I too was in the "alarming" position of having virtually no free memory. A quick glance at Task Manager revealed the truth. Though my "free" memory is indeed negligible, this is because so much is used by cache. The important number is not "free," but "available." The "available" memory includes both memory that is free, and memory that can be trivially made available, and this figure is far more representative of the true amount of memory available to applications. The vast majority of cached memory can be freed up near-instantly, since it is used up merely by cached data from disk.