i'm guessing that 501 ATK is just the sum from the numbers on your character window, since this is actually quite low as a "real" total ATK;
a +100 SP gear would slightly give more that just that since it gets modified by %SP gears;
so there's no way to know exactly w/out more info; in can be one or the other depending on the actual setup;
a rough idea is that ATK contributes more damage on the lower end of ATK+SP ← this essentially translates into "how good can you get the total setup to be", while as you increase this ATK+SP limit, SP tries to catch up in terms of damage contribution (but never catches up to ATK, but can get close to it);
players rely on SP gear rather heavily since it's usually easier to increase SP by a large amount than to increase ATK by a comparable amount that would produce equal results damage-wise;
factors that matter would be, base level, total of all stats, total of all %damage modifiers (ATK, racial, etc), total of all equip and weapon ATK, total of all +SP and %SP modifiers, etc
do you have the RWC ring w/ card and enchants already? if so, you could just try to borrow a d.ring from a friend/guildie to compare - easiest way to go
otherwise, you can do calculations on your own to get the real total ATK and mSP and just plug in to the gfist formula: ATK*[ 8 + (maxSP/10) ]