I wopuld say go with the gig or 768megs.... Really i thinkg you would probaly get around tehe same perfomance out of 768megs and 1 gig ( unless you are doing a lot of multi tasking Like photoshop,flash,dreamweaver,illustrator, cinmea 4d but i doubt yoi will do taht
Macs Only website tested a 1.25 ghz mini with 256, 512 and 1 gb of RAM. They concluded that 256 would be adequate for most uses, 512 showed some improvement but the cost of 1 gb was excessive compared to the change in results. Unless you do a lot of RAM-intensive work you'll probably be satisfied with 512.
Macs Only is wrong. With 256 and a few things running, it was swapping so bad my music skipped in Itunes while the drive worked hard trying to be 10 places at the same time. 256 would be on for email and web, but if you do any real work at all and play music in the background, go with 512. So far with 512 never had a problem but for my needs 1GB will be even better.