i have a 9800se 128bit also. just download the omega drivers and softmod it to a 9800. this will unlock all 8 pipes but the card's memory bandwidth will still remain 128bit. so you will essentially only get a 9500pro.
as for the r360 cores; yes, they overclock very well (at least on my card). on water the card did 460 gpu/ with 1.8v i can play doom3 at 510mhz and bench, artifact free, at 525. my card also has tsop mem and there is no chance that i can get it over 300mhz stable even with huge ramsinks. normally i just leave it at 280 from stock 250.
as for just flashing the bios. i don't think that you will see much/ any performance increase from flashing the card. but if you do flash the card, be sure to set the mem mhz to a stable speed (in the bios you are going to flash the card with) before flashing the card or you'll be in a worls of hurt.
if you want, i can hook you up with some vmods i found out/ did on my card.
with descet cooling and clock your looking at around 5k in '03, but don't expect too much from it though