I currently have about 450 accounts with 90% of them using mysql and 20% using cron jobs that eat up about several hours of work on a weekly basis.
Using about 30 gig of bandwidth. I'm on a reseller account right now and it appears that my sites go down frequently, are slow, or my cron jobs never execute fully. I may be growing out of the reseller account, but not sure if its just not my host.
If your sites are slow and going down, that's a sure sign that you need to go up a level with your hosting plans.. of course going dedi would be a more comfortable solution but if you can't afford that at the time beind then you can go with a VPS
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450 accounts on a reseller... wow. Why would liquidweb suggest a VPS, that's still overload IMO. You need your own dedicated box, quick.
I'm suprised you weren't kicked/forced off the reseller earlier. Bandwidth utilization is very low but still - 450 accounts is 450+ named entries, httpd entries, all the mail accounts that go with it putting lots of load where you don't think on a probably already loaded reseller box.