It should depend on the game being tried, PS2 games are like PC games, each with their own demands in hardware power.
Still that CPU is too slow for almost any game over there and not even speed hacks will help that much. Heavy overclock can help a bit but I wouldn't count on it.
PCSX2 is a very demanding application that has to 'translate' each and every PS2 instruction to something the PC understand, add to it the game own demand and the picture is ugly for older and weaker machines, not much that can be done about this.
Imagination is where we are truly real
I would try the lates 0.9.7 beta with the August 2010 Plugin pack. The newer GSdx revisions are more optimized and run faster.
CPU: i7 930 @ 2.8GHz(Everyday)/3.6GHz(Gaming)/4.0GHz(Encoding)
GPU: Geforce GTX 480 @ 850MHz/1001MHz
Motherboard: ASUS P6X58D-E
RAM: 2x3 6GB OCZ @ 1527MHz 8-8-8-24
FFX runs like this for me without speedhacks and the normal floating point rounding and clamping settings...
CPU: Pentium D 'Presler' 915 2.8 ghz 2x2MB L2 @ 3.5 ghz
GPU: eVGA [Nvidia] 8600GT 256MB SSC DDR3
Tested: FFX, FFX-2, FFXII, MGS3, KH, KH2, The Hobbit NTSC