the differens between pc 2700 and pc 3200 isnt too big but pc 3200 is at 400mhz and pc2700 is at 333mhz so 3200 is faster but it is slightly cheaper too so i'd go with the 3200.
and yes a pc3200 1gig 2x512 should be able to replace the pc 2700 512 but check your motherboard to make sure it is compatible with dual channel ram and if it is compatible with pc 3200. but if it is then you should be fine.
New build in progress, but this is what I got for now:
-AMD x2 5200+
-Asus M3A78-EM AM2+
-Buffalo 2x1gb ddr2 800
-Hitachi 250gb Sata HDD
-Antec Minuet Case w/ 350w psu
-Windows 7 Home Premium 64 bit