Hi, MATHA531
I don't have Sony's cell phone, so I will answer your question #2.
K610i (or K600i if you don't care for memory slot) may be a bit cheaper than K800i (when looking through google's product search).
K610i apparently worked in Japan; here is a web page for the report.
http://www.aetherwide.com/articles/roamjapan.html
I am not sure whether your roaming cards will work in Japan or not. The key is UMTS 2100.
Hope you will enjoy the upcoming trip to Japan. Per my family living in Japan, it has been VERY hot and humid in Japan (particularly bad this summer!).
Yazcedric
Cellphones: Nokia E70 (unlocked), Nokia 6010 (unlocked), Nokia 6030 (T-mobile USA)
Prepaid SIM: T-mobile USA, Smartone-Vodafone HK, Yackiemobile, O2 UK
Expired SIM: Orange France (prepaid)
Unknown status: MeSim (most likely expired)
Prepaid phone: Softbank Sharp 201SH
VoIP: Voxalot, voipcheap, Tpad.com, Freedigits.com, Globe7, Gizmo project, voicestick
Mobile VoIP: Truphone, Fring