u will encounter problems, kc its actually more advisable to use 2 ram modules(SPECIALLY if u plan to use ur mother board's DUAL CHANNEL CAPABLITY for better RAM performance and better PC use and Gameplay) that are:
1) same BRAND(but in most cases not really a big deal but some ram brands dnt play nice with each other specially the GENERIC ones)
2) same TYPE (SDRAM, DDR 1/2 )
3) same SIZE 128+128, 256+256, 512+512, 1gb+1gb, 2gb+2gb and so on
4) same SPEED (DDR266/333/400 and up)
immediate problems will not be experienced, its a longrun issue coz of the unbalanced amount of memory ur MoBo and CPU utilizes if u use different ram modules, its in the life span of ur hardware