GTS in SLI will certainly help in Oblivion, but I wouldn't drop an extra $400 for the few extra frames it'll provide.
Your 22 inch likely has a max resolution of 1680x1050. At 1600x1200 the GTS pulls about 100fps in Oblivion (with a C2D X6800). Dunno what ur using now for a CPU, but a GTS will likely give you at least 60fps in Oblivion at that resolution (1680x1050) with most eye candy on high.
So my answer is no, SLI with the GTS and GTX (right now) isn't worth it.
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