i rarely develop the GUI code under linux. use kylix several times. i pay more time on kernel. i am not familar with the cpu u said. especially the performance. i think u had better use a thin windows manager. for the code, i suggest u use python (if performance allow) since u can use vxwindows and then later u have code support both windows and linux.
for the server, if u are familiar with linux, u can use trustix or other linux version instead of rh9. or u can use rh as 3. i do not think rh9 has good performance. i think for this problem. u can ask people here who build and maintain this site, they have more experience than me in this field. i do not touch user space programming for almost 3 years.